Merge branch 'jk/test-match-signal' into maint

The test framework learned a new helper test_match_signal to
check an exit code from getting killed by an expected signal.

* jk/test-match-signal:
  t/lib-git-daemon: use test_match_signal
  test_must_fail: use test_match_signal
  t0005: use test_match_signal as appropriate
  tests: factor portable signal check out of t0005