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()

Lib/test/libregrtest/utils.py

@@ -6,6 +6,9 @@ import textwrap
 from test import support
 
 
+MS_WINDOWS = (sys.platform == 'win32')
+
+
 def format_duration(seconds):
     ms = math.ceil(seconds * 1e3)
     seconds, ms = divmod(ms, 1000)