git p4 test: use 'test_atexit' to kill p4d and the watchdog process

git p4 test: use 'test_atexit' to kill p4d and the watchdog process

Use 'test_atexit' to run cleanup commands to stop 'p4d' at the end of
the test script or upon interrupt or failure, as it is shorter,
simpler, and more robust than registering such cleanup commands in the
trap on EXIT in the test scripts.

Note that one of the test scripts, 't9801-git-p4-branch.sh', stops and
then re-starts 'p4d' twice in the middle of the script; take care that
the cleanup functions to stop 'p4d' are only registered once.

Note also that 'git p4' tests invoke different functions in the trap
on EXIT ('cleanup') and in the last test before 'test_done'
('kill_p4d').  Register both of these functions with 'test_atexit' for
now, and a a later patch in this series will then clean up the
redundancy.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>