Bring code coverage of immutable collections library up to ~97.5% block coverage

This PR fills in some gaps in test coverage for immutable collections, bringing block code coverage up from ~94.5% to ~97.5%.  This included adding new [Fact]s and fleshing out existing ones, adding the ImmutableArrayInteropTest.cs to the tests .csproj file (the file was in the repo but wasn't included in the test project for some reason), and removing some internal methods that were unused, e.g. Requires.NotNull(IntPtr, ...).