test-casecollision-merge: fix the test
D30 (055fee3547df) makes the error message unpredictable, therefore the fix.
Differential Revision: https://phab.mercurial-scm.org/D494
Greg Ward, author of the original bfiles extension
Na'Tosha Bard of Unity Technologies
Fog Creek Software
Special thanks to the University of Toronto and the UCOSP program