t6131 - skip tests if on case-insensitive file system

This test fails on Cygwin where the default system configuration does not
support case sensitivity (only case retention), so don't run the test on
such systems.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>