chore: run jest in two 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=2. One process took the suite past the
30-second cap in script/test, so two it is; the cap is unchanged.
make check, the pre-commit hook and script/cibuild all reach jest
through these scripts. The suite timings quoted 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=2",
+        "test:verbose": "jest --forceExit --maxWorkers=2 --verbose",
         "build": "node build.js",
         "lint": "eslint . && prettier --check .",
         "fmt": "prettier --write .",