Better integration of doctest with unittest and libregrtest

[serhiy-storchaka]

Feature or enhancement

A side effect of migration to the unittest test runner (a long process finished in #89392) was the change in the output in verbose mode. In older version, when test.support.run_doctest() was used to run doctests, it outputted all doctest details when regrtests are run with -v. Now doctests are wrapped in doctest.DocTestClass and only test name is shown when regrtests are run with -v.

The former behavior was too verbose, the current behavior is too terse. I want the following behavior:

• When regrtests are run without -v, do not output anything.
• When regrtests are run with single -v, output only doctest name, as for other test cases.
• When regrtests are run with -vv, output detailed doctest output.
• Every example count as a subtest.
• When some example fails, count it as failing subtest, and output detailed reports for every failed example. It should not include details of successful examples (except when run with -vv).
• It would be nice to integrate it also with the unittest CLI, but taking into account that it needs two -vs to show test case names and will need three -vs to show doctest details.

It means that we should found a way to pass verbosity level to doctests and run every example within TestCase.subTest().