Make test_translations test our tree

In order to run the correct classes, Python test framework adjusts
sys.path. However, these changes are not propagated to subprocesses.
Therefore, the test actually tries to test installed Swift, not
the one in which it is running.

The usual suggestion is to run "python setup.py develop" before
testing, but it's annoying and error-prone. If you forget it,
you may test the code in /usr very easily, and never know.

Let's just pass the correct path to subprocess. Much safer.

Change-Id: Ic71314e8462cf6e0579d704ffe9fbbfac7e6ba24
Merge "Make test_translations test our tree"