build-sys: meson: Give ourselves a little more timeout headroom

We're seeing CI failures due to once-test sometimes taking long. Let's
give ourselves a little more space per test (2 min -> 5 min) to avoid
this.

src/tests/meson.build

@@ -193,7 +193,7 @@ foreach t : default_tests + norun_tests
   if default_tests.contains(t)
     test(name, exe,
       env : test_env,
-      timeout : 120,
+      timeout : 300,
     )
   endif
 endforeach