docs(contributing): update testing section (#5657)

This was missed in the Mocha migration.

The options I've removed from the docs were removed from the code; I didn't think it worth implementing them in Mocha land until we definitely needed them. So shout if you miss any of the options!

Co-authored-by: Mathias Bynens <mathias@qiwi.be>

CONTRIBUTING.md

@@ -171,12 +171,11 @@ npm run unit -- --verbose
 npm run unit -- --break-on-failure
 ```
 
-- To run a specific test, substitute the `it` with `fit` (mnemonic rule: '*focus it*'):
+- To run a specific test, substitute the `it` with `it.only`:
 
 ```js
   ...
-  // Using "fit" to run specific test
-  fit('should work', async function({server, page}) {
+  it.only('should work', async function({server, page}) {
     const response = await page.goto(server.EMPTY_PAGE);
     expect(response.ok).toBe(true);
   });
@@ -205,25 +204,6 @@ HEADLESS=false npm run unit
 BINARY=<path-to-executable> npm run unit
 ```
 
-- To run tests in slow-mode:
-
-```bash
-HEADLESS=false SLOW_MO=500 npm run unit
-```
-
-- To run tests with additional Launcher options:
-
-```bash
-EXTRA_LAUNCH_OPTIONS='{"args": ["--user-data-dir=some/path"], "handleSIGINT": true}' npm run unit
-```
-
-
-- To debug a test, "focus" a test first and then run:
-
-```bash
-node --inspect-brk test/test.js
-```
-
 ## Public API Coverage
 
 Every public API method or event should be called at least once in tests. To ensure this, there's a `coverage` command which tracks calls to public API and reports back if some methods/events were not called.