If there's no p4 binary, the git-p4 tests would fail. This change makes
it skip instead.
Ran unit tests without p4 installed.
Summary
ID
Skip git-p4 tests if p4 is not on the system.
If there's no p4 binary, the git-p4 tests would fail. This change makes
it skip instead.
Testing Done:
Ran unit tests without p4 installed.
Fixes bug 4996.