feat(tests): test Edmonds-Karp failure in sparse mode

tests/edmondskarp.rs

@@ -167,12 +167,24 @@ fn unknown_source() {
     edmonds_karp_dense(&[1, 2, 3], &0, &3, Vec::<((i32, i32), i32)>::new());
 }
 
+#[test]
+#[should_panic(expected = "source not found in vertices")]
+fn unknown_source_2() {
+    edmonds_karp_sparse(&[1, 2, 3], &0, &3, Vec::<((i32, i32), i32)>::new());
+}
+
 #[test]
 #[should_panic(expected = "sink not found in vertices")]
 fn unknown_sink() {
     edmonds_karp_dense(&[1, 2, 3], &1, &4, Vec::<((i32, i32), i32)>::new());
 }
 
+#[test]
+#[should_panic(expected = "sink not found in vertices")]
+fn unknown_sink_2() {
+    edmonds_karp_sparse(&[1, 2, 3], &1, &4, Vec::<((i32, i32), i32)>::new());
+}
+
 fn str_to_graph(desc: &str) -> (Vec<usize>, Vec<Edge<usize, isize>>) {
     let vertices = (0..desc.lines().count() - 1).collect();
     let edges = desc