Rewrite apps test to ensure children are killed

Create a function run_with_application that both ensures the
app is running exactly while the test is running, which most
of the tests use.  We start it beforehand, and kill it after.
This avoids having any interdependence between the tests (and
there definitely was before, because we didn't wait for
the process to actually terminate after a kill() call).

Also, open a pipe between the two, and have the child app
monitor that pipe.  If it gets closed (e.g. because the parent
died), the child exits.  This is the most reliable way to
avoid stale children; before, if we failed an assertion, the
parent would abort, and not run kill().

https://bugzilla.gnome.org/show_bug.cgi?id=621034