gh-109162: libregrtest: add Logger class (#109212)

* Add Logger class in a new logger.py file.
* Move Regrtest attributes to Logger:

  * start_time
  * test_count_text
  * test_count_width
  * win_load_tracker

* Move Regrtest method to Logger:

  * log()
  * getloadavg(): rename to get_load_avg()
  * set_tests()

* Add methods to the Logger class:

  * start_load_tracker()
  * stop_load_tracker()