Add tests to reproduce a ValueError when x500UniqueIdentifier is of type UTF8String

[vxgmichel]

Add tests to reproduce issue #228

[wbond]

Could you rebase this on master?

[vxgmichel]

Could you rebase this on master?

Sure, I allowed edits by maintainers 👍

[wbond]

A few things:

1. The code doesn't pass linting
2. purest isn't used on this project
3. The test harness from this project requires specifying a new test class in test_classes() of tests/__init__.py, so currently these tests aren't run, and why it wasn't obvious that pytest isn't supported

[vxgmichel]

@wbond here you go, all your points should be addressed : 3758f73 :)

[vxgmichel]

Thank you @wbond for fixing #268

And yes this is also a shameless ping for issue #228 :)