Unskip unit test AddImportTests.TestAddImportWithCaseChange

Following unit test is attempting to validate that the CodeActionPriority of the offered code fix is Medium and not High. https://github.com/dotnet/roslyn/blob/aaed1133a5bc0db1387ae8538f0b648520d55241/src/EditorFeatures/VisualBasicTest/Diagnostics/AddImport/AddImportTests.vb#L78-L102

However, due to a bug in test framework, the validation is not even being performed. Attempting to fix the test framework in https://github.com/dotnet/roslyn/pull/41363 causes the unit test to fail as the offered code fix is indeed still at CodeActionPriority.High. I am going to skip the unit test as part of that PR, and use this issue to track fixing the product behavior and unskip the test.