Allow running each firedrake test as a script

If a test file is run as a script using python rather than py.test,
call the pytest main to achieve the same result.
Make py.test respects some environment variables

PYTEST_{VERBOSE,EXITFIRST,NOCAPTURE,TBNATIVE,WATCH} are translated
into py.test options:

* PYTEST_VERBOSE -> -v
* PYTEST_EXITFIRST -> -x
* PYTEST_NOCAPTURE -> -s
* PYTEST_TBNATIVE -> --tb=native
* PYTEST_WATCH -> -f (requires the pytest-xdist plugin)