[Piglit] Don't allow test names to be reassigned accidentally

[Dylan Baker]

This series removes all duplicate test entries (or examples of tests
being reassigned to a different test).

The big change is that this changes the profile.test_list to not allow
tests to be reassigned normally, if one does, an exception will be
raised (and caught to print a nice error message). If one wants to
actually override a test name then one can use the
profile.allow_reassignment context manager to explicitly reassign tests.
See quick.py for an example of this functionality.