The tonic method currently returns `///Users/test/foo` instead of `file:///Users/test/foo`.

Found while looking into making isolate spawning for flutter_tester work.

…quit (#47684)

GtkApplication windows contain a reference back to the application. This
means the MyApplication object in the Flutter application never
disposes. We workaround this by removing these references before we
quit. This occurs for all GTK applications that quit using this method,
see https://gitlab.gnome.org/GNOME/gtk/-/issues/6190. This may be fixed
upstream at some point but the proposed workaround should solve the
problem for all versions and not conflict with any upstream solution.

Fixes flutter/flutter#136582

This test worked by 1) releasing an item to be destructed on a background thead 2) adding a second item to this queue with a waitable event.

The idea being you could wait for the event and that would tell you when 1) was complete. Unfortunately these items are released by being placed into a std::vector, which destroyed in _reverse_ order. Somtimes when the test state was queried the work was done and sometimes it wasn't

To fix this, add a second waitable event that guarantees that both the first event and the original event have finished - because we do not add these event until the first is destructed and we do not add items to the destroy list while it is in progress.