regrtest_class.patch: Convert the long and complex main() function into a new Regrtest class using attributes and methods.

* Convert main() variables to Regrtest attributes: see the __init__() method. I documented some attributes

* Convert accumulate_result() function to a method

* Create setup_python() and setup_regrtest() methods. Maybe both methods should be merged, but for the first change I preferred to keep almost the same instruction order (not move code too much).

* Import gc at top level: the --threshold command line option is now ignored if the gc module is missing.

* Move resource.setrlimit() and the code to make the module paths absolute into the new setup_python() method. So this code is no more executed when the module is imported, only when main() is executed. We have a better on when the setup is done.

* Move textwrap import from printlist() to the top level.

* Some other minor cleanup.