[APM] Update testing dev doc (elastic#119804) (elastic#120469)

* [APM] Update testing dev doc

* Update x-pack/plugins/apm/dev_docs/testing.md

Co-authored-by: Cauê Marcondes <55978943+cauemarcondes@users.noreply.github.com>

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>
Co-authored-by: Cauê Marcondes <55978943+cauemarcondes@users.noreply.github.com>

Co-authored-by: Kate Patticha <kate@kpatticha.com>
Co-authored-by: Cauê Marcondes <55978943+cauemarcondes@users.noreply.github.com>

x-pack/plugins/apm/dev_docs/testing.md

@@ -23,17 +23,44 @@ API tests are separated in two suites:
 - a basic license test suite [default]
 - a trial license test suite (the equivalent of gold+)
 
+### Run tests with [--trial] license
+
 ```
 node scripts/test/api [--trial] [--help]
 ```
 
+The above command will initiate an Elasticsearch instance on http://localhost:9220 and a kibana instance on http://localhost:5620 and will run the api test against these environments.
+Once the tests finish, the instances will be terminated.
+
+### Start test server
+
+```
+node scripts/test/api --server
+```
+Start Elasticsearch and Kibana instances.
+
+### Run all tests
+
+```
+node scripts/test/api --runner
+```
+Run all tests. The test server needs to be running, see [Start Test Server](#start-test-server).
+
+### Update snapshots (from Kibana root)
+
+To update snapshots append `--updateSnapshots` to the `functional_test_runner` command
+
+```
+node scripts/functional_test_runner --config x-pack/test/apm_api_integration/[basic | trial]/config.ts --quiet --updateSnapshots
+```
+The test server needs to be running, see [Start Test Server](#start-test-server).
+
 The API tests are located in [`x-pack/test/apm_api_integration/`](/x-pack/test/apm_api_integration/).
 
 **API Test tips**
 
 - For data generation in API tests have a look at the [elastic-apm-synthtrace](../../../../packages/elastic-apm-synthtrace/README.md) package
 - For debugging access Elasticsearch on http://localhost:9220 and Kibana on http://localhost:5620 (`elastic` / `changeme`)
-- To update snapshots append `--updateSnapshots` to the functional_test_runner command
 
 ---