tests: simplify by pruning unwanted changesets a little earlier

The pruned commits are not relevant to the test case, so let's clean
them up at the end of the previous test case instead of part-way
through the next one.
2 jobs for topic/default/simplify-tests in 12 minutes and 44 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed flake8 #46157

00:00:16

passed tests #46158

00:12:44