Issue #25220: Split the huge main() function of libregrtest.main into a class

Issue #25220: Split the huge main() function of libregrtest.main into a class
with attributes and methods.

The --threshold command line option is now ignored if the gc module is missing.

* Convert main() variables to Regrtest attributes, document some attributes
* Convert accumulate_result() function to a method
* Create setup_python() function and setup_regrtest() method.
* Import gc at top level
* Move resource.setrlimit() and the code to make the module paths absolute into
  the new setup_python() function. So this code is no more executed when the
  module is imported, only when main() is executed. We have a better control on
  when the setup is done.
* Move textwrap import from printlist() to the top level.
* Some other minor cleanup.