Merge pull request #6818 from ckeditor/ckeditor5-dev/i/637-config-port

Docs: Document how to use a custom port for the automated test server.

docs/framework/guides/contributing/testing-environment.md

@@ -26,6 +26,7 @@ It accepts the following arguments that must be passed after the `--` option:
 * `--files` – Specifies test files to run. Accepts a package name or a glob. For example `--files=engine` will run tests from `ckeditor5-engine` package. Read more about the [rules for converting the `--files` option to a glob pattern](https://github.com/ckeditor/ckeditor5-dev/tree/master/packages/ckeditor5-dev-tests#rules-for-converting---files-option-to-glob-pattern).
 * `--browsers` – Browsers that will be used to run the tests. Defaults to `Chrome`.
 * `--debug` (alias `-d`) – Allows specifying custom debug flags. For example, the `--debug engine` option uncomments the `// @if CK_DEBUG_ENGINE //` lines in the code. Note that by default `--debug` is set to `true` even if you did not specify it. This enables the base set of debug logs (`// @if CK_DEBUG //`) which should always be enabled in the testing environment. You can completely turn off the debug mode by setting the `--debug false` option.
+* `--port` – Specifies the port for the server to use. Defaults to `9876`.
 
 ### Examples