Document the on-demand run of MySQL tests

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

doc/development/testing_guide/best_practices.md

@@ -2,19 +2,29 @@
 
 ## Test Design
 
-Testing at GitLab is a first class citizen, not an afterthought. It's important we consider the design of our tests 
-as we do the design of our features. 
+Testing at GitLab is a first class citizen, not an afterthought. It's important we consider the design of our tests
+as we do the design of our features.
 
-When implementing a feature, we think about developing the right capabilities the right way, which helps us 
-narrow our scope to a manageable level. When implementing tests for a feature, we must think about developing 
-the right tests, but then cover _all_ the important ways the test may fail, which can quickly widen our scope to 
+When implementing a feature, we think about developing the right capabilities the right way, which helps us
+narrow our scope to a manageable level. When implementing tests for a feature, we must think about developing
+the right tests, but then cover _all_ the important ways the test may fail, which can quickly widen our scope to
 a level that is difficult to manage.
 
-Test heuristics can help solve this problem. They concisely address many of the common ways bugs 
-manifest themselves within our code. When designing our tests, take time to review known test heuristics to inform 
-our test design. We can find some helpful heuristics documented in the Handbook in the 
+Test heuristics can help solve this problem. They concisely address many of the common ways bugs
+manifest themselves within our code. When designing our tests, take time to review known test heuristics to inform
+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
@@ -184,11 +194,11 @@ instead of 30+ seconds in case of a regular `spec_helper`.
 ### `let` variables
 
 GitLab's RSpec suite has made extensive use of `let`(along with it strict, non-lazy
-version `let!`) variables to reduce duplication. However, this sometimes [comes at the cost of clarity][lets-not], 
+version `let!`) variables to reduce duplication. However, this sometimes [comes at the cost of clarity][lets-not],
 so we need to set some guidelines for their use going forward:
 
 - `let!` variables are preferable to instance variables. `let` variables
-  are preferable to `let!` variables. Local variables are preferable to 
+  are preferable to `let!` variables. Local variables are preferable to
   `let` variables.
 - Use `let` to reduce duplication throughout an entire spec file.
 - Don't use `let` to define variables used by a single test; define them as
@@ -199,8 +209,8 @@ so we need to set some guidelines for their use going forward:
 - Try to avoid overriding the definition of one `let` variable with another.
 - Don't define a `let` variable that's only used by the definition of another.
   Use a helper method instead.
-- `let!` variables should be used only in case if strict evaluation with defined 
-  order is required, otherwise `let` will suffice. Remember that `let` is lazy and won't 
+- `let!` variables should be used only in case if strict evaluation with defined
+  order is required, otherwise `let` will suffice. Remember that `let` is lazy and won't
   be evaluated until it is referenced.
 
 [lets-not]: https://robots.thoughtbot.com/lets-not