t5601-99: Enable colliding file detection for MINGW

Commit b878579a (clone: report duplicate entries on case-insensitive
filesystems - 2018-08-17) adds a warning to user when cloning a repo
with case-sensitive file names on a case-insensitive file system.

This test has never been enabled for MINGW.
It had been working since day 1, but I forget to report that to the
author.
Enable it after a re-test.

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5601-clone.sh

@@ -628,7 +628,7 @@ test_expect_success 'clone on case-insensitive fs' '
 	)
 '
-test_expect_success !MINGW,CASE_INSENSITIVE_FS 'colliding file detection' '
+test_expect_success CASE_INSENSITIVE_FS 'colliding file detection' '
 	grep X icasefs/warning &&
 	grep x icasefs/warning &&
 	test_i18ngrep "the following paths have collided" icasefs/warning