Add verification tests for Mocha's --extension option

javascript/mocha-recursive-extensions-test/README.md

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+# Tests to verify Mocha's `--extension` behaviour
+
+According to [Mocha's own docs](https://mochajs.org/#-extension-ext-watch-extensions-ext) one should be able to specify which extensions mocha will automatically load. 
+
+> Specifying --extension will remove .js as a test file extension; use --extension js to re-add it. For example, to load .mjs and .js test files, you must supply --extension mjs --extension js.
+
+It quite clearly says that by specifying an extension, the default should be cleared. I have never seen this happen :-(
+
+## Output contradicting the docs
+
+The [`test`](./test) directory only contains a single file with the extension `bar`, so this means only one file should be loaded according to the docs. Instead, four files are loaded:
+```
+mocha --extension bar
+
+This is not a test: no-test.js
+
+
+  normal.bar
+    ✓ should do nothing but register the test
+
+  normal.bar.js
+    ✓ should do nothing but register the test
+
+  normal.js
+    ✓ should do nothing but register the test
+```