test-lib: use more compact expression in PIPE prerequisite

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>
diff --git a/t/test-lib.sh b/t/test-lib.sh
index 8a91fb1..c730c0b 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -1062,14 +1062,8 @@
 
 test_lazy_prereq PIPE '
 	# test whether the filesystem supports FIFOs
-	case $(uname -s) in
-	CYGWIN*|MINGW*)
-		false
-		;;
-	*)
-		rm -f testfifo && mkfifo testfifo
-		;;
-	esac
+	test_have_prereq !MINGW,!CYGWIN &&
+	rm -f testfifo && mkfifo testfifo
 '
 
 test_lazy_prereq SYMLINKS '