t/test-lib.sh: make ULIMIT_FILE_DESCRIPTORS available to tests

In t1400 the prerequisite 'ULIMIT_FILE_DESCRIPTORS' is defined and used
to effectively guard the helper function 'run_with_limited_open_files'
from being used on systems that do not satisfy this prerequisite.

In the subsequent patch, we will introduce another test outside of t1400
that would benefit from using this prerequisite. So, move it to
'test-lib.sh' instead so that it can be used by multiple tests.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1400-update-ref.sh

@@ -1342,15 +1342,6 @@ test_expect_success 'fails with duplicate ref update via symref' '
 	test_cmp expect actual
 '
 
-run_with_limited_open_files () {
-	(ulimit -n 32 && "$@")
-}
-
-test_lazy_prereq ULIMIT_FILE_DESCRIPTORS '
-	test_have_prereq !MINGW,!CYGWIN &&
-	run_with_limited_open_files true
-'
-
 test_expect_success ULIMIT_FILE_DESCRIPTORS 'large transaction creating branches does not burst open file limit' '
 (
 	for i in $(test_seq 33)

t/test-lib.sh

@@ -1618,6 +1618,15 @@ test_lazy_prereq ULIMIT_STACK_SIZE '
 	run_with_limited_stack true
 '
 
+run_with_limited_open_files () {
+	(ulimit -n 32 && "$@")
+}
+
+test_lazy_prereq ULIMIT_FILE_DESCRIPTORS '
+	test_have_prereq !MINGW,!CYGWIN &&
+	run_with_limited_open_files true
+'
+
 build_option () {
 	git version --build-options |
 	sed -ne "s/^$1: //p"