Make test suite operable from a personal repo.
If cloning BBI from The Workshop using DVCS features
(e.g. using the perforce_software-p4bbi_dev remote
spec), the test suite would fail when parsing the
gnarly P4PORT value used by a personal repo. Though
P4CONFIG was unset, a value for P4CONFIG was still
being implicitly read from .p4enviro file.
The fix: Set P4ENVIRO to a junk value in bbi.sh, so as
to avoid accidentally interacting with a personal DVCS
repo which slickly applies a P4CONFIG value even after
we unset it, by reading it from the P4ENVIRO file
(~/.p4enviro by default).