tests: split out separate test for issue5020 The test was added to the existing setup in 41f6af50c0d8 (merge: fix crash on criss cross merge with dir move and delete (issue5020), 2017-01-31). I'm about to make a change that affects that test and it's much easier to follow if the test case for issue5020 is a separate test case. The separate test case is based on what mpm provided in comment 12 on the issue. `hg diff -r 41f6af50c0d8^ tests/test-merge-criss-cross.t` after this patch is pretty small (besides the added test). It's probably easier for reviewers to look at that than to try to understand the diff itself (I don't understand it). Differential Revision: https://phab.mercurial-scm.org/D6243