Also fixed a bug with the signals test, which didn't properly exercise
Tini: rather than check that Tini was properly exiting with 128 +
signal, it raced against Tini and was only successful if Tini didn't get
the change to spawn a subprocess!
Using the child subreaper mechanism, we can actually run
tests inside the CI environment without depending on Docker.
While this does not replace the existing tests, it allows
at least some functional coverage within CI.