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