Allow tests to pass in headless environment
Currently the test require a running graphical environment to pass.
Please make this requirement optional.
1/2 tests-libgitg OK 0.22 s
2/2 tests-gitg FAIL 0.22 s (killed by signal 6 SIGABRT)
Ok: 1
Expected Fail: 0
Fail: 1
Unexpected Pass: 0
Skipped: 0
Timeout: 0
--- stderr ---
(process:840451): Gtk-CRITICAL **: 10:37:23.300: gtk_style_context_add_provider_for_screen: assertion 'GDK_IS_SCREEN (screen)' failed
-------
2/2 tests-gitg FAIL 0.22 s (killed by signal 6 SIGABRT)
--- command ---