gtestutils: Handle empty argv array passed to g_test_init()

This can happen if a caller (ab)uses `execve()` to execute a gtest
process with an empty `argv` array. `g_test_init()` has to gracefully
handle such a situation.

Fix a few problem areas in the code, and add a simple test which checks
that `g_test_init()` doesn’t crash when called with an empty `argv`.

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

glib/gtester.c

@@ -668,8 +668,8 @@ parse_args (gint    *argc_p,
         }
     }
   /* collapse argv */
-  e = 1;
-  for (i = 1; i < argc; i++)
+  e = 0;
+  for (i = 0; i < argc; i++)
     if (argv[i])
       {
         argv[e++] = argv[i];