Make the tests easier to read and maintain

[curquiza]

⚠️ Git does not make things easy to review at all, maybe check the test file directly

Here are what I did:

• Add missing tests in unit tests configuration
• Move the global settings tests at the end of the file
• Make class variable to avoid repetition
• Add attributesForFaceting in the Settings struct
• The settings tests for the get methods (ex: getRankingRules) only call the get method. No need to update settings to test the settings fetch, we can check we get the default values
• Make the settings tests for the update method (ex: updateRankingRules) simplier: no need to check two updates in the same tests. It make things harder to maintain and to read.