run-tests.php: class for test file loading

This moves a bunch of code outside of run_tests(), making it a bit
more manageable. Additionally, accessors provide better readability
than isset() and friends.

This is a minimal patch that moves the code but does not refactor
much. For the sake of reviewing experience, it does not involve
further refactoring which could include:
* Removing setSection()
* Fixing up the mess with hasSection() vs. sectionNotEmpty(), only
  one of which is really needed.
* Moving more repetitive code into the new class.
All of this will be done with later commits.

Closes GH-6678.

Author: MaxSem