tests: don't rely on strerror text when testing rmdir failure

tests: don't rely on strerror text when testing rmdir failure

AIX doesn't make a distiction between EEXIST and ENOTEMPTY; relying
on the strerror string for the rmdir failure is fragile. Just test
that the start of the string matches the Git controlled "failed to
rmdir..."  error. The exact text of the OS generated error string
isn't important to the test.

Signed-off-by: Charles Bailey <cbailey32@bloomberg.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>