gh-109162: libregrtest: add Logger class

[vstinner]

• 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()

• Issue: Refactor test.libregrtest #109162