tests: Make perf tests non-parallel

We currently hard code the display name, and running multiple instances
at the same time will probably just make things too slow for CI.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2623>

tests/meson.build

@@ -64,6 +64,7 @@ foreach perf_test : perf_tests
       '--headless',
       '--perf=@0@'.format(perf_test),
     ],
+    is_parallel: false,
     env: perf_testenv,
   )
 endforeach