testsuite: give pre_cmd for mhu-perf 5x time
In !12468 (closed) we bumped mhu-perf
timeout. But it may still timeout in pre_cmd
in unregisterised jobs (e.g. https://gitlab.haskell.org/ghc/ghc/-/jobs/1854083), therefore this patch bumps pre_cmd
to 5x from the previous 2x to increase pipeline robustness.