Pin Node and don't rely on --test scanning a directory

node --test only searches a directory given as a positional from v25, so
CI's Node 24 tried to load test/assets as a module. Pass the files
instead, which works on any version.

Pin the version in .tool-versions alongside Ruby, so CI and a local
checkout agree on one place.

Author: simon04

.github/workflows/test.yml

@@ -26,7 +26,7 @@ jobs:
     - name: Set up Node
       uses: actions/setup-node@820762786026740c76f36085b0efc47a31fe5020 # v7.0.0
       with:
-        node-version: lts/*
+        node-version-file: .tool-versions
         cache: npm
     - run: npm ci
     - name: Typecheck the assets

.tool-versions

@@ -1 +1,2 @@
 ruby 4.0.6
+nodejs 26.8.2

package.json

@@ -2,7 +2,7 @@
   "name": "devdocs",
   "private": true,
   "scripts": {
-    "test": "node --test test/assets/",
+    "test": "node --test test/assets/*_test.js",
     "typecheck": "tsc -p tsconfig.json && tsc -p test/assets/tsconfig.json"
   },
   "devDependencies": {