Fixes#6850
This PR makes the last corrections needed to run all unit tests on Windows:
add a function to check if a hook is executable in a cross-platform compatible way
handle correctly the PATH surgery for Windows during hook execution
handle correctly an account compatibility over both ACMEv1 and ACMEv2
remove (finally!) the @broken_on_windows decorator.
* Fix account_tests
* Fix hook executable test
* Remove the temporary decorator @broken_on_windows
* Fix util_test
* No broken unit test on Windows anymore
* More elegant mock
* Fix context manager
* Adapt coverage
* Corrections
* Adapt coverage
* Forbid os.access