chore(eslint-config): disallow .only to be used in tests (#185)

#### What this PR does / why we need it:

As per title.

packages/eslint-config/index.js

@@ -1,5 +1,5 @@
 module.exports = {
-  plugins: ['simple-import-sort', 'check-file', 'unused-imports'],
+  plugins: ['simple-import-sort', 'check-file', 'unused-imports', 'no-only-tests'],
   extends: ['airbnb-base', 'airbnb-typescript/base', 'prettier', 'plugin:anti-trojan-source/recommended'],
   ignorePatterns: ['dist'],
   rules: {
@@ -70,6 +70,11 @@ module.exports = {
           'Do not use ambiguous identifiers like `id`, use context identifiers like `userId` or `postId` instead.',
       },
     ],
+
+    /**
+     * disallow .only to be used in tests
+     */
+    'no-only-tests/no-only-tests': 'error',
   },
   env: {
     node: true,

packages/eslint-config/package.json

@@ -18,6 +18,7 @@
     "eslint-plugin-anti-trojan-source": "^1.1.1",
     "eslint-plugin-check-file": "^2.4.0",
     "eslint-plugin-import": "^2.27.5",
+    "eslint-plugin-no-only-tests": "^3.1.0",
     "eslint-plugin-simple-import-sort": "^10.0.0",
     "eslint-plugin-unused-imports": "^2.0.0"
   }