Move `addNode`/`addEdge` tests away from getters (#60)

Summary:
Nothing big; these were just organized wrong.

wchargin-branch: test-reorg

src/backend/graph.test.js

@@ -257,6 +257,29 @@ describe("graph", () => {
         ).toBeUndefined();
       });
 
+      it("gets all nodes", () => {
+        const expected = [heroNode(), bananasNode(), crabNode(), mealNode()];
+        const actual = advancedMealGraph().getAllNodes();
+        expectSameSorted(expected, actual);
+      });
+
+      it("gets all edges", () => {
+        const expected = [
+          pickEdge(),
+          grabEdge(),
+          cookEdge(),
+          bananasIngredientEdge(),
+          crabIngredientEdge(),
+          eatEdge(),
+          crabLoopEdge(),
+          duplicateCookEdge(),
+        ];
+        const actual = advancedMealGraph().getAllEdges();
+        expectSameSorted(expected, actual);
+      });
+    });
+
+    describe("creating nodes and edges", () => {
       it("forbids adding a node with existing address", () => {
         expect(() =>
           simpleMealGraph().addNode({
@@ -295,27 +318,6 @@ describe("graph", () => {
         });
       });
 
-      it("gets all nodes", () => {
-        const expected = [heroNode(), bananasNode(), crabNode(), mealNode()];
-        const actual = advancedMealGraph().getAllNodes();
-        expectSameSorted(expected, actual);
-      });
-
-      it("gets all edges", () => {
-        const expected = [
-          pickEdge(),
-          grabEdge(),
-          cookEdge(),
-          bananasIngredientEdge(),
-          crabIngredientEdge(),
-          eatEdge(),
-          crabLoopEdge(),
-          duplicateCookEdge(),
-        ];
-        const actual = advancedMealGraph().getAllEdges();
-        expectSameSorted(expected, actual);
-      });
-
       // For the next two test cases: we're documenting this behavior,
       // though we're not sure if it's the right behavior. Perhaps we want
       // the namespaces to be forced to be disjoint. In that case, we can