test: run jest in one process, fail on an open handle (closes #766)

Jest force-stopped worker processes that could not exit within 500 ms on
a starved CPU, failing runs where every test passed. The test scripts now
pass --runInBand. script/lib/jest.js fails the run at once when jest
warns that it did not exit one second after the run, instead of
hanging. The Dockerfile test stage drops its 90-second timeout.

tests/jestRunner.test.js runs the runner on a fixture that leaves an
interval running and expects exit status 1.

Model: opus-5-5

Dockerfile

@@ -18,9 +18,8 @@ COPY . .
 RUN yarn run lint
 RUN script/check-censored
 
-# Test phase, same shape and for the same reason: the jest suite (its worker
-# cap is in package.json), rerun verbose on failure, then
-# script/test-verify-build.
+# Test phase, same shape and for the same reason: the jest suite, rerun
+# verbose on failure, then script/test-verify-build.
 #
 # node:22-slim (22.x LTS), 2026-02-24
 FROM node@sha256:5373f1906319b3a1f291da5d102f4ce5c77ccbe29eb637f072b6c7b70443fc36 AS test
@@ -33,9 +32,9 @@ RUN script/bootstrap
 
 COPY . .
 
-RUN timeout 90 yarn run test || \
+RUN yarn run test || \
     { echo "--- Rerunning with --verbose for details ---"; \
-      timeout 90 yarn run test:verbose; exit 1; }
+      yarn run test:verbose; exit 1; }
 RUN script/test-verify-build
 
 # Development environment with the extension built, and the last stage: a