The page workflows started one global webServer and ran with one worker.
Each worker now owns its own tests/browser/server.py on an OS-chosen port,
with its own temp root and database, through a shared `test` in
tests/browser/fixtures.mjs. baseURL, and so page, request and
browser.newContext, point at that worker's server. A server that dies
fails the running test with its stderr tail and the next test gets a
fresh one. The worker count comes from PM_TEST_WORKERS, else 1 under CI,
else a quarter of the CPUs up to 8.
Chromium workflows in the pinned image: 510 tests in 606 s with one
worker on main, 511 (one new isolation spec) in 119 s with eight.
Vito Sansevero