[Monotone-devel] testsuite environment fixups, please review

[Justin Patrin]

These changes aren't huge and don't seem to be breaking any tests but
fix up the way that the environment is dealt with in tests. I'd like
some review in rev 83e72a1d8fb8e67868b291b0c26efdb2d6c3d55b.

Essentially I just make sure we're not overwriting the real old
environment value before saving it, save/restore the environment
before/after each test, and added an unset_var() method for lua. I
didn't commit to a branch because the changes were so small...let me
know if I should just commit on a  branch in the future.

--
Justin Patrin