Test timeouts in CI

Currently, my pipeline https://gitlab.dune-project.org/dominic/dune-perftool/-/jobs/15432 fails only because of a timeout. I would always argue that the test problem size should be decreased, but in this particulate case I know that it took @rhess quite a bit to find a configuration, where a matrix free solver without preconditioner is converging at all, so it would be nice to increase the timeout in the CI by a bit. On my laptop, the offending test takes 16.5 seconds so it should be doable on the test server with a mild increase of the limit (currently 20s).