t-reftable-tree: add test for non-existent key

In the current testing setup for tree_search(), the case for
non-existent key is not exercised. Improve this by adding a
test-case for the same.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/unit-tests/t-reftable-tree.c

@@ -47,6 +47,7 @@ static void t_tree_search(void)
 		check_pointer_eq(nodes[i], tree_search(&values[i], &root, &t_compare, 0));
 	}
 
+	check(!tree_search(values, &root, t_compare, 0));
 	tree_free(root);
 }