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As a possible starting point for this, I'll point to https://hg.python.org/cpython/file/02b81a82a57d/Lib/test/__main__.py
Now, I'd expected running that in a process that was *already* running regrtest to fail miserably (it would stomp all over itself).
But what we could potentially do is launch a *clean* subprocess, where the only thing it did was:
 from _testcapi import run_in_subinterp
 regrtest_in_subinterpreter = (
 """
 from test import regrtest
 regrtest.main_in_temp_cwd()
 """
 )
 run_in_subinterp(regrtest_in_subinterpreter)
I'd currently expect that to fail as well, but I think the failures might be enlightening :)
I'm also not sure we need to integrate this directly into the main regrtest test runner - it could just be a separate submodule invoked like "python -m test.subinterpretertest".
If we later decided to integrate it, then it could go behind a "-usubinterpreter" resource that invoked "test.subinterpretertest" in a subprocess.