chore: run jest in three worker processes (closes #426)

The test and test:verbose scripts ran jest with one worker per CPU core,
about 47 processes and 7-8 GiB per run on the shared 48-core build host.
They now pass --maxWorkers=3. On that host the suite takes 23-29s, inside
the unchanged 30-second cap in script/test but not by much: one test
file, tests/persistedFieldContract.test.js, takes most of it. One or two
workers went past the cap, so this departs from the issue's two-process
limit. make check, the pre-commit hook and script/cibuild all reach jest
through these scripts; the timings in the script/test and Dockerfile
comments are updated to match.

Model: opus-5-5

package.json

@@ -6,8 +6,8 @@
     "license": "GPL-3.0",
     "private": true,
     "scripts": {
-        "test": "jest --forceExit",
-        "test:verbose": "jest --forceExit --verbose",
+        "test": "jest --forceExit --maxWorkers=3",
+        "test:verbose": "jest --forceExit --maxWorkers=3 --verbose",
         "build": "node build.js",
         "lint": "eslint . && prettier --check .",
         "fmt": "prettier --write .",