t3200: fix failure on case-insensitive filesystems

62d94a3a (t3200: Add test demonstrating minor regression in 41c21f2;
2013-09-08) introduced a test which creates a directory named 'a',
however, on case-insensitive filesystems, this action fails with a
"fatal: cannot mkdir a: File exists" error due to a file named 'A' left
over from earlier tests. Resolve this problem.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>