test-lib: don't use ulimit in test prerequisites on cygwin

test-lib: don't use ulimit in test prerequisites on cygwin

On cygwin (and MinGW), the 'ulimit' built-in bash command does not have
the desired effect of limiting the resources of new processes, at least
for the stack and file descriptors. However, it always returns success
and leads to several test prerequisites being erroneously set to true.

Add a check for cygwin and MinGW to the prerequisite expressions, using
a 'test_have_prereq !MINGW,!CYGWIN' clause, to guard against using ulimit.
This affects the prerequisite expressions for the ULIMIT_STACK_SIZE,
CMDLINE_LIMIT and ULIMIT_FILE_DESCRIPTORS prerequisites.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>