Add troubleMakers inclusions to FE testing docs

What does this MR do?

Coverage testing pipeline failures caused by babel-plugin-rewire can be excluded by adding the problematics files to the trouble makers list in spec/javascripts/test_bundle.js.

This is however not noted in our documentation. The current advised approach is to rather add empty default exports on the getters and actions related to the file which shows up in the coverage report.

The MR changes the documented approach on how we handle preventing these test failures.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team