test: execute tests serially to speed up test execution (#284)

By default, `jest` runs tests in parallel. However, with the current
configuration of `ts-jest`, all workers run `tsc` in parallel, which is
very slow and consumes a lot of resources. This is what takes the
longest.
By running `jest` serially, `tsc` runs before starting the first test,
then all subsequent tests run faster. This will probably be improved by
customizing the configuration of `jest` or with future versions of
`ts-jest`.

packages/core/package.json

@@ -32,7 +32,7 @@
     "build": "tsc --version && tsc",
     "docs:api": "typedoc src/index.ts",
     "prepack": "run-s clean build",
-    "test": "jest"
+    "test": "jest --runInBand"
   },
   "devDependencies": {
     "jest": "^29.3.1",