gh-119050: Add XML support to libregrtest refleak checker (#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.