gtestutils: Allow clean simple use of g_test_trap_subprocess()

Allow g_test_trap_subprocess() to be used in a simple cases by
rerunning the same test case itself. This is accomplished by
passing %NULL as the test case name.

https://bugzilla.gnome.org/show_bug.cgi?id=720236
Simplify subprocesses in tests

Use the new way of running tests in a subprocess without
registering extra 'subprocess' test cases where appropriate.