tests: adapt now that Mercurial clears the merge state for empty wdir

hg evolve forgot to clear the merge state when its attempt to commit resulted in no commit because the working copy had no changes. That resulted in test-evolve-issue5967.t showing a conflict state that shouldn't be there. The responsibility for clearing the merge state was moved into Mercurial in Mercurial in 6a34e438461b (commit: clear resolved mergestate even if working copy is clean, 2020-02-28), so the bug is now fixed in hg evolve, which impacts that test case.