python: make it easier to run a limited number of tests

* lang/python/tests/Makefile.am: prefer py_tests from the environment
if present.

I'm trying to make it nicer/quicker to hack on the testsuite for
python bindings. With this change, if you're improving the python
bindings test suite, you can selectively run only a few specific tests
like so:

lang/python$ make check py_tests='t-decrypt.py t-decrypt-verify.py'

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>