Fix test_stats_arenas_summary fragility.
Fix test_stats_arenas_summary fragility.

Fix test_stats_arenas_summary to deallocate before asserting that
purging must have happened.

leebaok/jemalloc-4.2.1-readcode,AndroidExternal/jemalloc,leebaok/jemalloc-4.2.1-readcode,AndroidExternal/jemalloc,cpeterso/jemalloc,jemalloc/jemalloc-cmake,leebaok/jemalloc-4.2.1-readcode,8l/jemalloc,cpeterso/jemalloc,cpeterso/jemalloc,glandium/jemalloc,8l/jemalloc,wqfish/jemalloc,cpeterso/jemalloc,wqfish/jemalloc,jemalloc/jemalloc-cmake,glandium/jemalloc,jemalloc/jemalloc-cmake,AndroidExternal/jemalloc,8l/jemalloc,wqfish/jemalloc,AndroidExternal/jemalloc,glandium/jemalloc,glandium/jemalloc,wqfish/jemalloc,jemalloc/jemalloc-cmake,8l/jemalloc,wqfish/jemalloc,leebaok/jemalloc-4.2.1-readcode