Upgrade the python task tests to use TaskTestBase.

Review Request #2031 — Created April 4, 2015 and submitted — Latest diff uploaded

jsirois, zundel
1) Upgrades the PythonTaskTest to extend TaskTestBase instead of TaskTest.
2) Make all relevant tests subclass PythonTaskTest.
3) Move a bunch of common code into that base class.

I removed some rather hacky code whose ostensible purpose was to
make the code under test use the test-invoking code's InterpreterCache.
It would have been hard to make this code work under the new scheme,
but in any case, no benchmarks I ran could detect any performance
difference before and after. Even if I kept the old code but just
commented out that hack I could see no performance difference.

Let's revisit this performance issue if it crops up, or feel free
to show me that this is necessary and I'll figure something out.

CI passes: https://travis-ci.org/pantsbuild/pants/builds/57187738