remove redudnant cast to IReadOnlyList and add a not null ckeck in AssemblyEnumeratorTests#5402

Youssef1313 merged 10 commits intomicrosoft:mainmicrosoft/testfx:mainfrom

SimonCropp:remove-redudnant-cast-to-IReadOnlyList-and-add-a-not-null-ckeck-in-AssemblyEnumeratorTestsSimonCropp/testfx:remove-redudnant-cast-to-IReadOnlyList-and-add-a-not-null-ckeck-in-AssemblyEnumeratorTestsCopy head branch name to clipboard