Document the new 'spec/fast_spec_helper.rb' file

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

doc/development/testing_guide/best_practices.md

@@ -90,6 +90,25 @@ Finished in 34.51 seconds (files took 0.76702 seconds to load)
 
 Note: `live_debug` only works on javascript enabled specs.
 
+### Fast unit tests
+
+Some classes are well-isolated from Rails and you should be able to test them
+without the overhead added by the Rails environment and Bundler's `:default`
+group's gem loading. In these cases, you can `require 'fast_spec_helper'`
+instead of `require 'spec_helper'` in your test file, and your test should run
+really fast since:
+
+- Gems loading is skipped
+- Rails app boot is skipped
+- Gitlab::Shell and Gitaly setup are skipped
+- Test repositories setup are skipped
+
+Note that in some cases, you might have to add some `require_dependency 'foo'`
+in your file under test since Rails autoloading is not available in these cases.
+
+This shouldn't be a problem since explicitely listing dependencies should be
+considered a good practice anyway.
+
 ### `let` variables
 
 GitLab's RSpec suite has made extensive use of `let` variables to reduce
@@ -281,14 +300,13 @@ All fixtures should be be placed under `spec/fixtures/`.
 
 RSpec config files are files that change the RSpec config (i.e.
 `RSpec.configure do |config|` blocks). They should be placed under
-`spec/support/config/`.
+`spec/support/`.
 
 Each file should be related to a specific domain, e.g.
-`spec/support/config/capybara.rb`, `spec/support/config/carrierwave.rb`, etc.
+`spec/support/capybara.rb`, `spec/support/carrierwave.rb`, etc.
 
-Helpers can be included in the `spec/support/config/rspec.rb` file. If a
-helpers module applies only to a certain kind of specs, it should add modifiers
-to the `config.include` call. For instance if
+If a helpers module applies only to a certain kind of specs, it should add
+modifiers to the `config.include` call. For instance if
 `spec/support/helpers/cycle_analytics_helpers.rb` applies to `:lib` and
 `type: :model` specs only, you would write the following:
 
@@ -299,6 +317,14 @@ RSpec.configure do |config|
 end
 ```
 
+If a config file only consists of `config.include`, you can add these
+`config.include` directly in `spec/spec_helper.rb`.
+
+For very generic helpers, consider including them in the `spec/support/rspec.rb`
+file which is used by the `spec/fast_spec_helper.rb` file. See
+[Fast unit tests](#fast-unit-tests) for more details about the
+`spec/fast_spec_helper.rb` file.
+
 ---
 
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