[3.13] gh-119050: Add XML support to libregrtest refleak checker (GH-119148) (#119270)

gh-119050: Add XML support to libregrtest refleak checker (GH-119148)

regrtest test runner: Add XML support to the refleak checker
(-R option).

* run_unittest() now stores XML elements as string, rather than
  objects, in support.junit_xml_list.
* runtest_refleak() now saves/restores XML strings before/after
  checking for reference leaks. Save XML into a temporary file.
(cherry picked from commit 9257731f5d3e9d4f99e314b23a14506563e167d7)

Co-authored-by: Victor Stinner <vstinner@python.org>