Define and document acceptable thresholds for rspec tests

Related discussion &8729 (comment 1103759355)

Using /rspec_profiling_stats to determine a threshold. We can start with the average number of the top 50 slow tests per file

• Average time for top 50 slow spec files: 577.0309535077 seconds
• Average time for top 50 slow individual tests: 79.41 seconds

This is 5% of the top slow 1000 tests.

With this issue, we have:

• the track of our decision on the threshold of the RSpec test
• document it under https://docs.gitlab.com/ee/development/testing_guide/best_practices.html#test-speed

Even if at first there is nothing preventing adding slower test, this could bring the first awareness for engineers

With next iterations, we can come up with thresholds for different example types. This MR could help