Update UseTests: Swallow unknown version errors

Now that the mock home directory isn't swallowing function errors, these
are resulting in test failures when they aren't actually intended.
In these cases, we use `#expect` to verify the version, but swallow any
errors returned by the swiftly invocation.

In all cases, these were the result of trying to switch to a toolchain
that didn't exist. This should emit an error message, but leave the
selected toolchain unchanged.

Author: etcwilde

Tests/SwiftlyTests/UseTests.swift

@@ -11,9 +11,17 @@ import Testing
     /// Execute a `use` command with the provided argument. Then validate that the configuration is updated properly and
     /// the in-use swift executable prints the the provided expectedVersion.
     func useAndValidate(argument: String, expectedVersion: ToolchainVersion) async throws {
-        try await SwiftlyTests.runCommand(Use.self, ["use", "-g", argument])
+        do {
+            try await SwiftlyTests.runCommand(Use.self, ["use", "-g", argument])
+        } catch {
+            // Swallow any errors. Some of these tests verify that switching to
+            // a non-existent toolchain does not change the version
+            // configuration.
+            // Swiftly should emit an error message, but we don't care.
+        }
 
-        #expect(try await Config.load().inUse == expectedVersion)
+        let useToolchain = try await Config.load().inUse
+        #expect(useToolchain == expectedVersion)
     }
 
     /// Tests that the `use` command can switch between installed stable release toolchains.
@@ -201,15 +209,20 @@ import Testing
     /// Tests that the `use` command gracefully exits when executed before any toolchains have been installed.
     @Test(.mockedSwiftlyVersion(), .mockHomeToolchains(toolchains: []))
     func useNoInstalledToolchains() async throws {
-        try await SwiftlyTests.runCommand(Use.self, ["use", "-g", "latest"])
+        do {
+            try await SwiftlyTests.runCommand(Use.self, ["use", "-g", "latest"])
 
-        var config = try await Config.load()
-        #expect(config.inUse == nil)
+            var config = try await Config.load()
+            #expect(config.inUse == nil)
 
-        try await SwiftlyTests.runCommand(Use.self, ["use", "-g", "5.6.0"])
+            try await SwiftlyTests.runCommand(Use.self, ["use", "-g", "5.6.0"])
 
-        config = try await Config.load()
-        #expect(config.inUse == nil)
+            config = try await Config.load()
+            #expect(config.inUse == nil)
+        } catch {
+            // Swallow errors. This test is verifying that we don't change the
+            // in-use toolchain if no toolchains are installed
+        }
     }
 
     /// Tests that the `use` command gracefully handles being executed with toolchain names that haven't been installed.