gtestutils: skipping a test should count as success, not failure

In particular, the test program as a whole should exit with status 0
if you skipped some tests but did not fail any.

https://bugzilla.gnome.org/show_bug.cgi?id=720263
gtestutils: rename test_skip_count to test_startup_skip_count

https://bugzilla.gnome.org/show_bug.cgi?id=720263
Make g_test_run() return 77 if all tests are skipped

Change g_test_run() to return 1 on failure (rather than the number of
failed tests), and 77 if all tests are skipped (since automake and
some other test harnesses recognize that status code).

Previously g_test_run() returned the number of failed tests, but this
behavior was not documented, and at any rate, prior to 2.39,
g_test_run() would normally not return at all if an error occurred.

https://bugzilla.gnome.org/show_bug.cgi?id=720263