Fix unittests: unique names for Pythran functions

It seems that under some conditions the validation fails
if there are test cases using the same function names.

The log is like:

E  ArgumentError: Python argument types in
E                 test_list_comparison.list_comparison(list, list)
E                 did not match C++ signature:
E  list_comparison(pythonic::core::list<pythonic::core::list<long> >)

pythran/tests/test_env.py:79: ArgumentError