TST: Make test_partial_iteration_cleanup robust but require leak checker
TST: Make test_partial_iteration_cleanup robust but require leak checker

This makes sure the test is not flaky, but the test now requires
a leak checker (both valgrind or reference count based should work
in CPython at least).

Closes gh-21169

charris/numpy,numpy/numpy,mattip/numpy,charris/numpy,numpy/numpy,charris/numpy,mattip/numpy,numpy/numpy,numpy/numpy,charris/numpy,mattip/numpy,mattip/numpy