I ran Bustle/dbus-monitor on the session bus while running the full GLib
test suite, and noticed that these tests were causing `Launched` signal
emissions on the main session bus. That suggested they weren’t isolated
properly, which at best causes noise on the bus and at worst could cause
spurious test failures.

Fix that by running those tests in a `GTestDBus` environment, as some of
the other appinfo tests already are.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>