Frontend coverage runs as three shards (PM_TEST_WORKERS=3 each) that upload
their raw V8 files as coverage-raw-N; a merge job downloads them, fails unless
all three arrived with raw files and no clashing names, then runs
coverage:merge. Firefox and WebKit page workflows run as two shards each with
three workers. The required viewerjs context keeps its name and now needs the
shards, the merge and the browser shards, failing unless every one succeeded.