testhelpers: fixed test for future Mercurial 6.0

This is not upstream in mercurial_testhelpers because it depends
on the `prune` command.
5 jobs for !69 with topic/default/compat-testhelpers in 3 minutes and 16 seconds (queued for 3 seconds)
latest detached
Status Name Job ID Coverage
  Main
passed tests-current #254328

00:01:35

100.0%
 
  Compat
passed check-sdist #254329

00:01:58

passed tests-hg-default #254331

00:01:33

100.0%
passed tests-hg-git-default #254332

00:01:08

100.0%
passed tests-hg-stable #254330

00:01:39

100.0%