What is the problem?
In the test environement, we were calling `SiteSetting.setting` directly
to introduce new site settings. However, this leads to changes in state of the SiteSettings
hash that is stored in memory as test runs. Changing or leaking states
when running tests is one of the major contributors of test flakiness.
An example of how this resulted in test flakiness is our `spec/integrity/i18n_spec.rb` spec file which
had a test case that would fail because a new "plugin_setting" site
setting was registered in another test case but the site setting did not
have translations for the site setting set.
What is the fix?
There are a couple of changes being introduced in this commit:
1. Make `SiteSetting.setting` a private method as it is not safe to be
exposed as a public method of the `SiteSetting` class
2. Change test cases to use existing site settings in Discourse instead
of creating custom site settings. Existing site settings are not
removed often so we don't really need to dynamically add new site
settings in test cases. Even if the site settings being used in test
cases are removed, updating the test cases to rely on other site
settings is a very easy change.
3. Set up a plugin instance in the test environment as a "fixture"
instead of having each test create its own plugin instance.
