Link to testing guide for feature flag spec info

In order for there to be a single source of truth.

doc/development/feature_flags.md

@@ -108,11 +108,11 @@ so we make sure behavior under feature flag doesn't go untested in some non-spec
 contexts.
 Whenever a feature flag is present, make sure to test _both_ states of the
-feature flag. You can stub a feature flag as follows:
-```ruby
-stub_feature_flags(my_feature_flag: false)
-```
+feature flag.
+See the
+[testing guide](testing_guide/best_practices.html#feature-flags-in-tests)
+for information and examples on how to stub feature flags in tests.
 ## Enabling a feature flag (in development)

doc/development/testing_guide/best_practices.md

@@ -240,6 +240,36 @@ it 'is overdue' do
 end
 ```
+### Feature flags in tests
+
+All feature flags are stubbed to be enabled by default in our Ruby-based
+tests.
+
+To disable a feature flag in a test, use the `stub_feature_flags`
+helper. For example, to globally disable the `ci_live_trace` feature
+flag in a test:
+
+```ruby
+stub_feature_flags(ci_live_trace: false)
+
+Feature.enabled?(:ci_live_trace) # => false
+```
+
+If you wish to set up a test where a feature flag is disabled for some
+actors and not others, you can specify this in options passed to the
+helper. For example, to disable the `ci_live_trace` feature flag for a
+specifc project:
+
+```ruby
+project1, project2 = build_list(:project, 2)
+
+# Feature will only be disabled for project1
+stub_feature_flags(ci_live_trace: { enabled: false, thing: project1 })
+
+Feature.enabled?(:ci_live_trace, project1) # => false
+Feature.enabled?(:ci_live_trace, project2) # => true
+```
+
 ### Pristine test environments
 The code exercised by a single GitLab test may access and modify many items of