tests: fix test-casefolding.t stable

tests: fix test-casefolding.t

The message had changed, but the test was not updated. This test does not
run on Linux, but failed on my Mac.

--- a/tests/test-casefolding.t	Wed Oct 19 15:07:11 2016 -0700
+++ b/tests/test-casefolding.t	Fri Oct 21 16:31:16 2016 +0100
@@ -51,7 +51,8 @@
   $ echo b > D/b
   $ hg ci -Am addb D/b
   $ hg mv D/b d/b
-  D/b: not overwriting - file exists
+  D/b: not overwriting - file already committed
+  (hg rename --force to replace the file by recording a rename)
   $ hg mv D/b d/c
   $ hg st
   A D/c