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

README.md

@@ -1761,8 +1761,8 @@ The `Makefile` targets are thin shims over `script/`:
   `package.json`.
 - `script/setup`: `script/bootstrap`, then the git pre-commit hook.
 - `script/test`: build the `test` stage of the `Dockerfile` with `--no-cache`.
-  The stage runs the jest suite, stopped at 90 seconds and rerun verbose on
-  failure, then `script/test-verify-build`.
+  The stage runs the jest suite, rerun verbose on failure, then
+  `script/test-verify-build`.
 - `script/lint`: build the `lint` stage with `--no-cache`. The stage runs
   ESLint, the prettier check and `script/check-censored`. Linting runs only in
   Docker.
@@ -1861,9 +1861,10 @@ have left a complete, loadable debug bundle behind. `make build-debug` keeps its
 
 ### Tests
 
-`make test` runs the jest suite in Docker. The suite must finish within 60
-seconds; the test stage stops it at 90. `make check` runs the tests, the lint
-stage and the formatting check.
+`make test` runs the jest suite in Docker, in one process. The suite must finish
+within 60 seconds. A test that leaves a timer or another open handle running
+fails the run. `make check` runs the tests, the lint stage and the formatting
+check.
 
 Some unit tests pin facts stated elsewhere in this file: