Return the exit code from tox to callers of run_tests.sh

Invoking PIPESTATUS appears to be volatile in my environment,
after the if condition it is already again 0, even if it was
non-zero before. Capture the result permanently in a variable
and use that one consistently.

Also handle the just-pep8 case correctly.

Change-Id: Icca8c00c8d2189da1a133db277c9071c2d6c880b