test helpers: move test-* to t/helper/ subdirectory

This keeps top dir a bit less crowded. And because these programs are
for testing purposes, it makes sense that they stay somewhere in t/

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-read-cache.c

@@ -0,0 +1,13 @@
+#include "cache.h"
+
+int main (int argc, char **argv)
+{
+	int i, cnt = 1;
+	if (argc == 2)
+		cnt = strtol(argv[1], NULL, 0);
+	for (i = 0; i < cnt; i++) {
+		read_cache();
+		discard_cache();
+	}
+	return 0;
+}