tests: Bump slow test timeout to 600 seconds

On our CI and package builders `test-update-install-flatpaks` often
exceeds the 360 second timeout. Even on my fast laptop it routinely
takes nearly 300 seconds. Bump the timeout for slow tests to 600 seconds
to ensure it has time to complete.

https://phabricator.endlessm.com/T5658

tests/meson.build

@@ -106,7 +106,7 @@ foreach test_name, extra_args : test_programs
     exe,
     env: envs,
     suite: ['eos-updater'] + extra_args.get('suite', []),
-    timeout: extra_args.get('slow', false) ? 360 : 60,
+    timeout: extra_args.get('slow', false) ? 600 : 60,
     is_parallel: extra_args.get('parallel', true),
   )
 endforeach