Update testing guide to reference test design

doc/development/testing_guide/best_practices.md

 # Testing best practices
 
+## 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. 
+
+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 Design](https://about.gitlab.com/handbook/engineering/quality/guidelines/test-engineering/test-design/) section.
+
 ## Test speed
 
 GitLab has a massive test suite that, without [parallelization], can take hours

doc/development/testing_guide/index.md

@@ -33,7 +33,7 @@ changes should be tested.
 
 ## [Testing best practices](best_practices.md)
 
-Everything you should know about how to write good tests: RSpec, FactoryBot,
+Everything you should know about how to write good tests: Test Design, RSpec, FactoryBot,
 system tests, parameterized tests etc.
 
 ---