ci(browser): rely on the step limit, report the running tests on SIGTERM

Maintainer ruling on #425: no Playwright globalTimeout. The 18-minute
step limit under the unchanged 20-minute job limit stays, as do the
always() uploads and the CI blob report.

- Drop globalTimeout and PM_TEST_GLOBAL_TIMEOUT_MINUTES with its helper
  and tests.
- A step that reaches its limit gets SIGTERM, which ended Playwright with
  no report and no blob. The running-tests reporter now catches the first
  SIGTERM, prints the tests still running and sends Playwright its own
  SIGINT, so the run ends interrupted and the blob report is written. A
  second SIGTERM kills as usual.
- Test paths are relative to the suite's rootDir instead of the cwd, so
  `npm run --prefix` no longer prints ../../ prefixes.
- The workflow comment and TESTING.md say the guarantee assumes an npm
  cache hit and that coverage shard 1 is tight in its 18-minute step.

.github/workflows/viewerjs-check.yml

@@ -28,12 +28,14 @@ concurrency:
 # 7m08s/4m34s), webkit 8m16s/5m03s (tests 7m26s/3m58s); a cache miss adds up to 5m to npm ci, which
 # the 20-minute limits absorb.
 # Hangs (#425): a job killed at its limit keeps no log and skips its upload steps, so each browser
-# test step has its own timeout-minutes (18) below the job's 20, and Playwright's globalTimeout
-# (PM_TEST_GLOBAL_TIMEOUT_MINUTES) stops the run before the step and lists the interrupted tests.
+# test step has its own timeout-minutes (18) below the job's 20. At the step limit, SIGTERM makes
+# tools/vendor/running-tests-reporter.mjs print the tests still running and hand over to Playwright's
+# interrupt, which writes the blob report; the always() uploads then run.
 # Measured on main (runs 40725289094, 40719375442, 40700439399): setup before the test step <= 52s,
-# uploads <= 35s; test steps up to 14m42s (webkit shard 1/2, Playwright ~14.6m) and 13m05s for
-# coverage (workflows 11.8m after ~1m of node units and the viewer suite, whose own CI limit is 3m).
-# So: page workflows globalTimeout 17m, coverage workflows 15m, each step 18m.
+# uploads <= 35s; test steps up to 14m42s (webkit shard 1/2) and 13m05s for coverage (shard 1/3:
+# 12m53s, node units and the viewer suite before 11.8m of workflows), so coverage is the tight one
+# in its 18 minutes. This holds only when the npm cache hits: a cold cache adds up to 5m to npm ci,
+# and then the job limit can still end the job before the step limit does.
 jobs:
   frontend-coverage:
     name: frontend coverage (shard ${{ matrix.shard }}/3)
@@ -87,8 +89,6 @@ jobs:
         run: npm run --prefix tools/vendor test
       - name: Page workflows with coverage, shard ${{ matrix.shard }}/3
         timeout-minutes: 18
-        env:
-          PM_TEST_GLOBAL_TIMEOUT_MINUTES: 15
         run: npm run --prefix tools/vendor test:coverage:shard -- ${{ matrix.shard }}/3
       - name: Upload raw coverage
         uses: actions/upload-artifact@v7
@@ -223,8 +223,6 @@ jobs:
       - run: npm ci --prefix tools/vendor --ignore-scripts
       - name: Page workflows in ${{ matrix.browser }}, shard ${{ matrix.shard }}/2
         timeout-minutes: 18
-        env:
-          PM_TEST_GLOBAL_TIMEOUT_MINUTES: 17
         run: npm run --prefix tools/vendor test:workflows -- --project ${{ matrix.browser }} --shard ${{ matrix.shard }}/2
       # Traces and the blob report (test-results/blob/) of a failed, timed-out or cancelled step.
       - name: Upload browser traces