Adding a No-op task executor for testing

Fixing DFS implementation of DAG validation to be much faster
Adding in additional tests to ensure the run order of expanded tasks is preserved
Adding additional compile-time checks, resolving issues that came up as a result

tests/unit_dag.cpp

@@ -11,7 +11,7 @@ TEST_CASE("dag_construction", "[dag]") {
     REQUIRE(dag.empty());
 
     REQUIRE_NOTHROW(dag.addVertex(0, 0));
-    for (int i = 1; i < 10; ++i) {
+    for (size_t i = 1; i < 10; ++i) {
         dag.addVertex(i, i);
         REQUIRE(dag.hasVertex(i));
         REQUIRE(dag.getVertex(i).data == i);
@@ -23,7 +23,7 @@ TEST_CASE("dag_construction", "[dag]") {
 
     // Cannot add an edge that would result in a cycle
     dag.addEdge(9, 5);
-    REQUIRE_THROWS(dag.isValid());
+    REQUIRE(!dag.isValid());
 
     // Bounds checking
     SECTION("addEdge Bounds Checking") {