Move UnitGenericAbstractType To Nunit (#7257)

Author: falfaddaghi

tests/fsharp/Compiler/ErrorMessages/UnitGenericAbstactType.fs

@@ -0,0 +1,27 @@
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace FSharp.Compiler.UnitTests
+
+open NUnit.Framework
+open FSharp.Compiler.SourceCodeServices
+
+[<TestFixture>]
+module ``Unit generic abstract Type`` =
+
+    [<Test>]
+    let ``Unit can not be used as return type of abstract method paramete on return type``() =    
+        CompilerAssert.TypeCheckSingleError
+            """
+type EDF<'S> =
+    abstract member Apply : int -> 'S
+type SomeEDF () =
+    interface EDF<unit> with
+        member this.Apply d = 
+            // [ERROR] The member 'Apply' does not have the correct type to override the corresponding abstract method.
+            ()
+            """
+            FSharpErrorSeverity.Error
+            17
+            (6, 21, 6, 26)
+            "The member 'Apply : int -> unit' is specialized with 'unit' but 'unit' can't be used as return type of an abstract method parameterized on return type."
+