fix compiler warnings: return 0 in main(), in 3 tests

Change-Id: I9e49ceba6b0a8dffd331e5707667d9bb007f3ec7
diff --git a/tests/lib/ippool_test.c b/tests/lib/ippool_test.c
index ea56edd..5b3227c 100644
--- a/tests/lib/ippool_test.c
+++ b/tests/lib/ippool_test.c
@@ -130,4 +130,5 @@
 	srand(time(NULL));
 
 	test_pool_sizes();
+	return 0;
 }