Move execution reordering to main iterator

[epdenouden]

Move all existing test execution reordering functionality to the code that answers the question what test shall we run next?

This refactoring is a continuation of the work for the recent @depends improvements and does not change anything for the end-users. The deliverable is a small pull request with @internal clean up.

Why

Folding the existing reordering mechanism into the core iterator and cleaning up the supporting code has tangible benefits:

• Reordering currently only happens on startup, giving the sorter a limited view. This is closely related to how @dataProvider tests are loaded.
• The TestSuiteSorter sorts the internal tests graph which can have side-effects. In contrast the extended iterator will simply loop over the existing tree in a different order. This makes it more flexible and robust.
• Lay the groundwork for a clean implementation of on-demand @dataProvider tests with proper rewind/next hooks.
• The refreshed code is easier to understand and maintain. The TestSuiteSorter has too much knowledge of the runner internals.

To do

• [x] create an additional minimal interface for tests that implement the requirements for the PHPUnit @depends-resolver
  • [x] implement ReorderableTest (name suggestions welcome!)
  • [x] implement getInternalId() for all PHPUnit test objects
  • [x] further improve provides-requires dependency resolver for all reordarable test types
• [ ] use TestSuiteIterator for all test iteration
  • [ ] extend iterator to keep to-do/done lists instead of just looping over a list
  • [ ] sort a TestSuite during Iterator::rewind()
  • [ ] add dependency checks to Iterator::next()
  • [ ] inject the result cache (see also: --filter mechanism)
  • [ ] inject test ordering options
• [ ] retire TestSuiteSorter as the main reordering mechanism
  • [ ] keep TestSuiteSorter comparators for sorting individual TestSuites
  • [x] ~move test graph traversal for calculation original execution order~ utility function fits here
  • [x] retire internal getSorterUID in favour of new interface
  • [ ] give unit tests for TestSuite[Iterator|Sorter] a good wash
• [ ] update internal test naming scheme in result cache
• [ ] get code to LTS release quality
  • [ ] full feature coverage
  • [ ] project coverage, especially remove obsolete reordering guards