make check-nosetests: Ignore user's PYTHONPATH

I had PYTHONPATH pointing at a different, installed, version of stbt and
it caused nose's test-collector to fail.

We already set PYTHONPATH for `make check-pylint`. Since 68983e0 we
neither set nor clear PYTHONPATH for `tests/run-tests.sh` (aka
`make check-integrationtests`) but I'm not sure whether I should leave
that as it is, or set `PYTHONPATH=` -- so I'll leave it as it is.
stbt camera selftests: Avoid spurious error message

Each test is run in its own temporary directory so in most cases this
file doesn't exist (however some tests call `start_fake_video_src` twice
so the `rm` *is* necessary).
stbt camera selftests: Cleanup `run_validation` arguments