testsuite: Perf stats are not saved for wasm builds

In push_perf_notes we see the conditional:

437   if [ -n "${CROSS_TARGET:-}" ] && [ "${CROSS_EMULATOR:-}" != "js-emulator" ]; then
438     info "Can't test cross-compiled build."                                     
439     return                                                                      
440   fi  

I think we can probably just remove this check as in .gitlab/test-metrics.sh, if the METRICS_FILE is not found then that just aborts anyway.

And the failure can be caught in push_perf_notes and refuse to completely fail the job in the case that pushing perf notes fails.

"$TOP/.gitlab/test-metrics.sh" push || info "Failed to push perf notes" 

However, this might silently hide other failures in pushing perf notes.

Perhaps the best immediate solution is to add another special case to this function for wasm backend.