Stop using symlinks.

When PYTHONDONTWRITEBYTECODE=x is not defined, test_env.pyc is created aside a symlink. The problem is that the real uses
os.path.realpath() to find its real path. On the second run, after a
test_env.pyc was created aside the symlink, this one is loaded and
not the real source file. This means that the source file path is different,
causing generalized failure.

(CL was reviewed manually, Rietveld couldn't handle replacing a symlink with a
7 files changed