Allow multiple utils modules on Shared Settings

[Belco90]

It would be interesting to allow "testing-library/utils-module" to support multiple modules. For doing so:

• "testing-library/utils-module" should expect an array of strings
• all tests cases setting "testing-library/utils-module" must be updated
• add new test cases in fake rule to test several "testing-library/utils-module" set
• update isAggressiveModuleReportingEnabled to check array length is greater than 0
• update detectionInstructions to check if some of the utils modules end with corresponding string.
• update migrating guide to reflect this change

This example makes it seem like this can only be a single string and not an array? Not sure if it'd add too much complexity to support multiple utils modules.

For example a user could have many custom selectors each imported from a different module. Probably not common, but just calling it out.

Originally posted by @CreativeTechGuy in #312 (comment)

[Belco90]

@CreativeTechGuy ok so I've been trying to implement the first approach for this but it's not so easy as I thought. Apart from changes mentioned in the description, all checks around custom test utils should be updated to look for specifiers on an array rather than a single string.

The plugin is not ready for this, since it assumes it will have a single import, not several of them. This implies a lot of work, including corresponding tests to make sure different cases with several imports work as expected.

For this reason, I think it's better to leave this as a future improvement for a 4.X release.

What I can do now is rename the setting to something like custom-utils so it's generic enough for receiving a single string or an array in the future.

[CreativeTechGuy]

Wow thank you for spending so much time on this suggestion. But I definitely agree, we should wait and see if there's any demand for it since it's a non-trivial change.

[Belco90]

No problem. I thought it would be a quick win, but it's not. I'm gonna close the ticket but reference it in v4 issue as a future improvement.

[rosczja]

I would definitely like to have this feature, but I understand that this is not trival to implement.

We have two different types of tests: unit and integration. Both use a different custom render function and re-export testing-library stuff. Each type has its own module.