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'