Add first couple of LLProcessLauncher tests.

Run INTEGRATION_TEST_llprocesslauncher using setpython.py so we can find the
Python interpreter of interest.
Introduce python() function to run a Python script specified using
NamedTempFile conventions.
Introduce a convention by which we can read output from a Python script using
only the limited pre-January-2012 LLProcessLauncher API. Introduce
python_out() function to leverage that convention.
Exercise a couple of LLProcessLauncher methods using all the above.