This is my first contribution to PHPUnit. I tried to follow convention best as I could including copying the copyright notice on the fixture test classes. Hope that's right.

This PR introduces a --order-by newest option for chronological test sorting, running the most recently modified test files first.

When developing tests in a project with a lot of existing tests, I wanted my new ones to go first. This new execution order option prioritizes recently modified test files. Combined with --stop-on-failure it can save time for a developer manually running tests.

Changes

• New execution order constant: TestSuiteSorter::ORDER_NEWEST
• CLI option: --order-by newest
• Sorting logic: Tests are ordered by file modified time

Design Decisions

• Implements newest-first ordering only, as oldest-first has no obvious (to me) use case
• Sorts at the test class file level rather than individual test methods (by necessity)
• Test suites are sorted by the modification time of the first test in the suite, I thought this was a reasonable answer for what to do with suites rather than a more complex check of the content

Testing

• Added end-to-end test with fixture files
• Test includes setup script to maintain consistent file timestamps

Usage

# Run tests with newest files first
phpunit --order-by newest