Document the on-demand run of MySQL tests

Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/development/testing_guide/best_practices.md

@@ -15,6 +15,16 @@ manifest themselves within our code. When designing our tests, take time to revi
 our test design. We can find some helpful heuristics documented in the Handbook in the
 [Test Design](https://about.gitlab.com/handbook/engineering/quality/guidelines/test-engineering/test-design/) section.
 
+## Run tests against MySQL
+
+By default, tests are only run againts PostgreSQL, but you can run them on
+demand against MySQL by following one of the following conventions:
+
+| Convention           | Valid example                |
+|:----------------------|:-----------------------------|
+| Include `mysql` in your branch name | `enhance-mysql-support` |
+| Include `[run mysql]` in your commit message   | `Fix MySQL support<br><br>[run mysql]` |
+
 ## Test speed
 
 GitLab has a massive test suite that, without [parallelization], can take hours