Dispose test env in RequiredTransformations

Fixes a possible test-time crash introduced in f8330fd: there's no
specified ordering for running the finalizer of _env, so sometimes it
might run after another Dispose cleaned up the test environment, and
then attempt to throw an assertion exception at a poorly-specified time,
leading to a crash in the test process instead of a test failure.

Author: rainersigwald

src/Tasks.UnitTests/ResourceHandling/GenerateResource_Tests.cs

@@ -20,7 +20,7 @@ namespace Microsoft.Build.UnitTests.GenerateResource_Tests.InProc
 {
     [Trait("Category", "mono-osx-failing")]
     [Trait("Category", "mono-windows-failing")]
-    public sealed class RequiredTransformations
+    public sealed class RequiredTransformations : IDisposable
     {
         private readonly TestEnvironment _env;
         private readonly ITestOutputHelper _output;
@@ -31,6 +31,11 @@ public RequiredTransformations(ITestOutputHelper output)
             _output = output;
         }
 
+        public void Dispose()
+        {
+            _env.Dispose();
+        }
+
         /// <summary>
         ///  ResX to Resources, no references
         /// </summary>