Implement repetition of files using pytest_collect_file hook: run
file collection as many times as needed to cover all --mode/--repeat
combinations. Also move disabled test logic to this hook.
Store build mode and run_id in pytest item stashes.
Simplify support of C++ tests: remove redundant facade abstraction and put
all code into 3 files: base.py, boost.py, and unit.py
Add support for `run_first` option in test_config.yaml
Some test suites have own test runners based on pytest, and they
don't need all stuff we use for test.py. Move all code related to
test.py framework to test/pylib/runner.py and use it as a plugin
conditionally (by using TEST_RUNNER variable.)