git-p4 tests: refactor and cleanup

git-p4 tests: refactor and cleanup

Introduce a library for functions that are common to
multiple git-p4 test files.

Be a bit more clever about starting and stopping p4d.
Specify a unique port number for each test, so that
tests can run in parallel.  Start p4d not in daemon mode,
and save the pid, to be able to kill it cleanly later.
Never kill p4d at startup; always shutdown cleanly.

Handle directory changes better.  Always chdir inside
a subshell, and remove any post-test directory changes.

Clean up whitespace, and use test_cmp and test_must_fail
more consistently.

Separate the tests related to detecting p4 branches
into their own file, and add a few more.

Acked-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>