Add --only task_id={number} to useful mix test options (#1531)

* Add `--only task_id={number}` to useful `mix test` options

* Update docs/TESTS.md

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Co-authored-by: Angelika Cathor <angelikacathor@fastmail.com>

docs/TESTS.md

@@ -129,6 +129,7 @@ Documentation:
   in the same order they were defined in
 * `--stale` - runs only tests which reference modules that changed since the last
   time tests were ran with `--stale`
+* `--only task_id:1` - or with another number, on learning exercises runs only the tests associated with the specific task
 
 Documentation: