t1402-check-ref-format: skip tests of refs beginning with slash on Windows

t1402-check-ref-format: skip tests of refs beginning with slash on Windows

Bash on Windows converts program arguments that look like absolute POSIX
paths to their Windows form, i.e., drive-letter-colon format. For this
reason, those tests in t1402 that check refs that begin with a slash do not
work as expected on Windows: valid_ref tests are doomed to fail, and
invalid_ref tests fail for the wrong reason (that there is a colon rather
than that they begin with a slash).

Skip these tests.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>