Issue #25220, libregrtest: Call setup_python(ns) in the slaves

Slaves (child processes running tests for regrtest -jN) now inherit
--memlimit/-M, --threshold/-t and --nowindows/-n options.

* -M, -t and -n are now supported with -jN
* Factorize code to run tests.
* run_test_in_subprocess() now pass the whole "ns" namespace to the child
  process.