Explicitly test physics/quantum/qubit/qubit_values

The unit test for Qubit doesn't explicitly test qubit_values (they are tested for IntQubit). Add an assertion for this

(I'm not sure how fine-grained the owners like pull requests - I have some other changes to make to test_qubit.py which are really just refactorings to make it easier to read, I'll add them as a separate pull request)

Give me a yell if there are any issues with this, ta