tests/kvm: Don't run in parallel

They can be quite heavy, as they load up one virtual machine each. If
your system is already busy, this can easily cause them to time out
instead of finish in time, as they all fight over the same limited
amount of CPU and I/O time.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2556>

src/tests/kvm/meson.build

@@ -48,6 +48,7 @@ foreach test_case: privileged_tests
       test_case['executable'].full_path(),
       meson.current_build_dir(),
       vm_env,
-    ]
+    ],
+    is_parallel: false,
   )
 endforeach