[3.12] gh-83434: Sync libregrtest and test_regrtest with the main branch (#117250)

[3.12] gh-83434: Sync libregrtest and test_regrtest with the main branch (#117250)

* gh-115122: Add --bisect option to regrtest (#115123)

* test.bisect_cmd now exit with code 0 on success, and code 1 on
  failure. Before, it was the opposite.
* test.bisect_cmd now runs the test worker process with
  -X faulthandler.
* regrtest RunTests: Add create_python_cmd() and bisect_cmd()
  methods.

(cherry picked from commit 1e5719a663d5b1703ad588dda4fccd763c7d3e99)

* gh-115720: Show number of leaks in huntrleaks progress reports (GH-115726)

Instead of showing a dot for each iteration, show:
- '.' for zero (on negative) leaks
- number of leaks for 1-9
- 'X' if there are more leaks

This allows more rapid iteration: when bisecting, I don't need
to wait for the final report to see if the test still leaks.

Also, show the full result if there are any non-zero entries.
This shows negative entries, for the unfortunate cases where
a reference is created and cleaned up in different runs.

Test *failure* is still determined by the existing heuristic.

(cherry picked from commit af5f9d682c20c951b90e3c020eeccac386c9bbb0)

* gh-83434: Disable XML in regrtest when -R option is used (#117232)

(cherry picked from commit d52bdfb19fadd7614a0e5abaf68525fc7300e841)

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Co-authored-by: Petr Viktorin <encukou@gmail.com>