browser.test_pthread_asan_use_after_free_2_wasmfs hangs on Firefox

https://github.com/emscripten-core/emscripten/pull/20005 fixes that test for WasmFS, and it passes in Chrome,
but it hangs in Firefox. That is quite strange as WasmFS proxies less, so in general it
should have fewer hang bugs.

I investigated a little to rule out obvious issues. The page is actually hanging on CPU work,
so it isn't a reporting issue (it's not that the browser harness isn't getting a result), and
a reporting change is all that PR does, so I'm pretty sure that test was failing in Firefox
due to 2 reasons earlier, and that PR fixes 1 of the 2.