test-doc-portal: Work around GTestDbus env unsetting

tests/test-doc-portal.c

@@ -201,6 +201,9 @@ main (int argc, char **argv)
   g_test_dbus_add_service_dir (dbus, TEST_SERVICES);
   g_test_dbus_up (dbus);
 
+  /* g_test_dbus_up unsets this, so re-set */
+  g_setenv ("XDG_RUNTIME_DIR", outdir, TRUE);
+
   g_spawn_command_line_sync (DOC_PORTAL " -d", NULL, NULL, &exit_status, &error);
   g_assert_cmpint (exit_status, ==, 0);
   g_assert_no_error (error);