Switch test framework to PyTest

[Prof Garth Wells]

Following the DOLFIN change, switch test system to PyTest.

[Juan Luis Cano]

I tried to simply replace python test.py by py.test at least for unit tests and got an error:

+ py.test
======================================= test session starts ========================================
platform linux2 -- Python 2.7.11, pytest-2.8.1, py-1.4.30, pluggy-0.3.1
rootdir: /home/juanlu/miniconda2/conda-bld/work/ffc-1.5.0, inifile: 
collected 0 items / 1 errors 

============================================== ERRORS ==============================================
_______________ ERROR collecting test/unit/evaluate_basis/test_against_ref_values.py _______________
evaluate_basis/test_against_ref_values.py:22: in <module>
    from .cppcode import evaluate_basis_code
E   ImportError: cannot import name evaluate_basis_code
===================================== 1 error in 0.75 seconds ======================================
TESTS FAILED

[Martin Sandve Alnæs]

Suggested path: when merging uflacs into ffc, move uflacs tests into ffc, then transition ffc unit tests to pytest (not too much work), then consider what to do with regression tests. Would be nice to have those within the same framework.

[Anders Logg (Chalmers)]

Sounds good.