build: test code must run in older node.js

We must disable eslint rules that require new javascript features in
test files, because test files are not currently compiled by `tsc`.

TODO: doesn't look like babel is configured to run on the "neovim"
package. I tried this:

    M packages/neovim/package.json
    @@ -72,7 +72,7 @@
         "test-staged": "npm test --bail --no-cache --findRelatedTests",
         "test-missing-apis": "npm run build && node scripts/findMissingApi",
         "precommit": "lint-staged",
    -    "build": "tsc --pretty",
    +    "build": "tsc --pretty && babel lib --out-dir lib",
         "dev": "npm run build --watch true"
       },
       "jest": {
    M packages/neovim/src/utils/getNvimFromEnv.ts
    @@ -149,6 +149,8 @@ export function getNvimFromEnv(
       const paths = process.env.PATH.split(delimiter);
       const pathLength = paths.length;
       const matches = new Array<NvimVersion>();
    +  const foo = matches.at(-1);
    +  console.log(foo); // test
       const unmatchedVersions = new Array<NvimVersion>();
       const errors = new Array<GetNvimFromEnvError>();
       for (let i = 0; i !== pathLength; i = i + 1) {

Babel didn't translate the file to the desired node.js 8 compatible
syntax (it should replace `matches.at()` with `matches[…]`).

.eslintrc.js

@@ -16,6 +16,22 @@ module.exports = {
     es2024: true,
   },
 
+  overrides: [
+    {
+      files: ['*.test.ts'],
+      // excludedFiles: ['bin/*.ts', 'lib/*.ts'],
+      rules: {
+        // This rule requires es2022(?) but the CI node14 job runs the tests
+        // in node14, but the test code is not compiled/polyfilled... so the
+        // test code needs to be compatible with node14.
+        // TODO: can the back-compat CI jobs for older node.js versions run
+        // `jest` against the compiled .js results (would require compiling
+        // the test files as well)?
+        'unicorn/prefer-at': 'off',
+      }
+    }
+  ],
+
   rules: {
     curly: 'error', // Enforce braces on "if"/"for"/etc.
     // Avoid accidental use of "==" instead of "===".

packages/neovim/src/api/Tabpage.test.ts

@@ -65,7 +65,7 @@ describe('Tabpage API', () => {
       const tabpages = await nvim.tabpages;
       expect(tabpages.length).toBe(2);
 
-      nvim.tabpage = tabpages.at(-1);
+      nvim.tabpage = tabpages[tabpages.length - 1];
 
       const newTabPage = await nvim.tabpage;
       expect(await newTabPage.number).toBe(2);
@@ -139,7 +139,7 @@ describe('Tabpage API', () => {
       // TODO
       expect((await nvim.tabpages).length).toBe(2);
 
-      nvim.tabpage = tabpages.at(-1);
+      nvim.tabpage = tabpages[tabpages.length - 1];
 
       expect(await nvim.tabpage.number).toBe(2);
     });