Switch to depth-first traversal of TestSuites
Switch to depth-first traversal of TestSuites

More advanced sortorders require statistics about sub-TestSuites.
Traversing the tree of suites depth-first provides a simple way of
gathering data about underlying suites.

Example: sorting the tests defects-first requires statistics of the
total number/type of defects in any underlying tests.

Firehed/phpunit,sebastianbergmann/phpunit