Fix race condition in gapplication/basic test

Fix race condition in gapplication/basic test

The three processes this test creates need to be executed
in order, and g_usleep was used to guarantee that.
However, under heavy load, that is not enough. Instead,
wait until the children start by making sure they have
written to stdout before proceeding any further.

https://bugzilla.gnome.org/show_bug.cgi?id=664627