Make ConditionalFieldsFormHelper testable

Problem/Motivation

\Drupal\conditional_fields\ConditionalFieldsFormHelper modifies the form array with #states to provide the end-user functionality of the conditional_fields module. So it's very important. And should be tested so changes to its functionality do not cause regressions.

However, to even make ConditionalFieldsFormHelper testable, we need to make some changes to it.

Proposed resolution

Modify \Drupal\conditional_fields\ConditionalFieldsFormHelper as necessary to make its processDependentFields() and processDependeeFields() functions testable. Add tests for those functions.

Remaining tasks

1. Write a patch
2. Review and feedback
3. RTBC and feedback
4. Commit

User interface changes

None.

API changes

To be determined. Some methods in ConditionalFieldsFormHelper will need to change signature and visibility. However, I'll make an attempt not to break backwards compatibility, i.e.: by API additions, or giving new parameters default values so old code continues to work. Still might be worth merging this in a 2.x branch.

Data model changes

None.