[TASK] bamboo: Hard kill php web server processes

bamboo acceptance test shut down sometimes fails to stop
the 'php -S' web server process for unknown reasons.
A next job on the same agent then fails to bind to the
port, leading to broken builds.
Switch from SIGTERM to SIGKILL during test shut down
to see if that improves the situation.

Resolves: #84143
Releases: master, 8.7
Change-Id: I838a2341f1f8e25fa461b8747337406391b52241
Reviewed-on: https://review.typo3.org/56013


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>