Test framework update to more robustly clean up leftover files and
processes after tests are done.
* sg/test-atexit:
t9811-git-p4-label-import: fix pipeline negation
git p4 test: disable '-x' tracing in the p4d watchdog loop
git p4 test: simplify timeout handling
git p4 test: clean up the p4d cleanup functions
git p4 test: use 'test_atexit' to kill p4d and the watchdog process
t0301-credential-cache: use 'test_atexit' to stop the credentials helper
tests: use 'test_atexit' to stop httpd
git-daemon: use 'test_atexit` to stop 'git-daemon'
test-lib: introduce 'test_atexit'
t/lib-git-daemon: make sure to kill the 'git-daemon' process
test-lib: fix interrupt handling with 'dash' and '--verbose-log -x'