feat: add Fable.Python.Testing module for cross-platform testing

Add a Testing module that provides test utilities for writing tests that
run on both .NET (with XUnit) and Python (with pytest).

Features:
- `equal` / `notEqual` - Assert equality/inequality (F# style: expected first)
- `throwsError` - Assert exact error message
- `throwsErrorContaining` - Assert error contains substring
- `throwsAnyError` - Assert any error is thrown
- `doesntThrow` - Assert no error is thrown
- `FactAttribute` - Mark test functions for pytest discovery

Uses `Fable.Core.Testing.Assert` (the standard Fable way) instead of
custom Emit statements.

Also removes the local test/Util.fs and updates all test files to use
the new Fable.Python.Testing module directly.

Release-As: 5.0.0-alpha.21.4

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: dbrattli

src/Fable.Python.fsproj

@@ -38,6 +38,10 @@
     <Compile Include="fastapi/FastAPI.fs" />
 
     <Compile Include="fable/Types.fs" />
+    <Compile Include="fable/Testing.fs" />
+  </ItemGroup>
+  <ItemGroup Condition="'$(FABLE_COMPILER)' != 'true'">
+    <PackageReference Include="xunit" Version="2.9.*" />
   </ItemGroup>
   <ItemGroup>
     <Content Include="pyproject.toml; *.fsproj; **\*.fs; **\*.fsi" PackagePath="fable\" />

src/fable/Testing.fs

@@ -0,0 +1,79 @@
+/// Cross-platform test utilities for writing tests that run on both .NET (with XUnit) and Python (with pytest).
+///
+/// Example usage:
+/// ```fsharp
+/// open Fable.Python.Testing
+///
+/// [<Fact>]
+/// let ``test addition works`` () =
+///     let result = 2 + 2
+///     result |> equal 4
+///
+/// [<Fact>]
+/// let ``test throws on invalid input`` () =
+///     throwsAnyError (fun () -> failwith "boom")
+/// ```
+module Fable.Python.Testing
+
+open System
+
+#if FABLE_COMPILER
+open Fable.Core.Testing
+
+/// Assert equality (expected first, then actual - F# style)
+let equal expected actual : unit = Assert.AreEqual(actual, expected)
+
+/// Assert inequality
+let notEqual expected actual : unit = Assert.NotEqual(actual, expected)
+
+/// Attribute to mark test functions (compiles to test_ prefix for pytest)
+type FactAttribute() =
+    inherit System.Attribute()
+
+#else
+open Xunit
+
+/// Assert equality (expected first, then actual - F# style)
+let equal<'T> (expected: 'T) (actual: 'T) : unit = Assert.Equal(expected, actual)
+
+/// Assert inequality
+let notEqual<'T> (expected: 'T) (actual: 'T) : unit = Assert.NotEqual(expected, actual)
+
+/// FactAttribute is already provided by XUnit
+type FactAttribute = Xunit.FactAttribute
+#endif
+
+// Exception testing helpers (work on both platforms)
+
+let private run (f: unit -> 'a) =
+    try
+        f () |> Ok
+    with e ->
+        Error e.Message
+
+/// Assert that a function throws an error with the exact message
+let throwsError (expected: string) (f: unit -> 'a) : unit =
+    match run f with
+    | Error actual when actual = expected -> ()
+    | Error actual -> equal expected actual
+    | Ok _ -> equal expected "No error was thrown"
+
+/// Assert that a function throws an error containing the expected substring
+let throwsErrorContaining (expected: string) (f: unit -> 'a) : unit =
+    match run f with
+    | Error _ when String.IsNullOrEmpty expected -> ()
+    | Error (actual: string) when actual.Contains expected -> ()
+    | Error actual -> equal (sprintf "Error containing '%s'" expected) actual
+    | Ok _ -> equal (sprintf "Error containing '%s'" expected) "No error was thrown"
+
+/// Assert that a function throws any error
+let throwsAnyError (f: unit -> 'a) : unit =
+    match run f with
+    | Error _ -> ()
+    | Ok _ -> equal "An error" "No error was thrown"
+
+/// Assert that a function does not throw
+let doesntThrow (f: unit -> 'a) : unit =
+    match run f with
+    | Ok _ -> ()
+    | Error msg -> equal "No error" (sprintf "Error: %s" msg)

test/Fable.Python.Test.fsproj

@@ -12,7 +12,6 @@
     <ProjectReference Include="../src/Fable.Python.fsproj" />
   </ItemGroup>
   <ItemGroup>
-    <Compile Include="Util.fs" />
     <Compile Include="TestAst.fs" />
     <Compile Include="TestAsyncIO.fs" />
     <Compile Include="TestBuiltins.fs" />
@@ -26,6 +25,7 @@
     <Compile Include="TestPydantic.fs" />
     <Compile Include="TestFastAPI.fs" />
     <Compile Include="TestTypes.fs" />
+    <Compile Include="TestTesting.fs" />
     <Compile Include="Main.fs" />
   </ItemGroup>
   <Import Project="..\.paket\Paket.Restore.targets" />

test/TestAst.fs

@@ -1,6 +1,6 @@
 module Fable.Python.Tests.Ast
 
-open Util.Testing
+open Fable.Python.Testing
 open Fable.Python.Ast
 
 [<Fact>]

test/TestAsyncIO.fs

@@ -1,6 +1,6 @@
 module Fable.Python.Tests.AsyncIO
 
-open Util.Testing
+open Fable.Python.Testing
 open Fable.Python.AsyncIO
 
 [<Fact>]

test/TestBase64.fs

@@ -1,6 +1,6 @@
 module Fable.Python.Tests.Base64
 
-open Util.Testing
+open Fable.Python.Testing
 open Fable.Python.Base64
 open Fable.Python.Builtins
 

test/TestBuiltins.fs

@@ -1,6 +1,6 @@
 module Fable.Python.Tests.Builtins
 
-open Util.Testing
+open Fable.Python.Testing
 open Fable.Python.Builtins
 open Fable.Python.Os
 

test/TestFastAPI.fs

@@ -1,6 +1,6 @@
 module Fable.Python.Tests.TestFastAPI
 
-open Fable.Python.Tests.Util.Testing
+open Fable.Python.Testing
 
 #if FABLE_COMPILER
 open Fable.Core

test/TestJson.fs

@@ -1,6 +1,6 @@
 module Fable.Python.Tests.Json
 
-open Util.Testing
+open Fable.Python.Testing
 open Fable.Python.Json
 
 // Test types for union and record serialization

test/TestLogging.fs

@@ -1,6 +1,6 @@
 module Fable.Python.Tests.Logging
 
-open Util.Testing
+open Fable.Python.Testing
 open Fable.Python.Logging
 
 [<Fact>]