test: make FILEMODE a lazy prereq

This way, test authors don't need to remember to source
lib-prereq-FILEMODE.sh before using the FILEMODE prereq to guard tests
that rely on the executable bit being honored when checking out files.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/test-lib.sh b/t/test-lib.sh
index b25249e..5968157 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -830,6 +830,10 @@
 	ln -s x y && test -h y
 '
 
+test_lazy_prereq FILEMODE '
+	test "$(git config --bool core.filemode)" = true
+'
+
 test_lazy_prereq CASE_INSENSITIVE_FS '
 	echo good >CamelCase &&
 	echo bad >camelcase &&