Switch from using useRef() to createRef() (#1603)

## Summary:

I noticed a test that was jumping through hoops with spies in order to manage a ref in tests. Using `React.createRef()` is simpler and avoids needing to mock a hook. 

Issue: "none"

## Test plan:

`yarn test` ✅

Author: jeremywiebe

Reviewers: aemandine, #perseus, nishasy

Required Reviewers:

Approved By: aemandine

Checks: ❌ codecov/project, ✅ codecov/patch, ✅ Upload Coverage (ubuntu-latest, 20.x), ✅ Publish npm snapshot (ubuntu-latest, 20.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 20.x), ✅ Check builds for changes in size (ubuntu-latest, 20.x), ✅ Jest Coverage (ubuntu-latest, 20.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ✅ Cypress (ubuntu-latest, 20.x), 🚫 Upload Coverage, ✅ gerald, ✅ gerald, 🚫 Publish npm snapshot (ubuntu-latest, 20.x), 🚫 Cypress (ubuntu-latest, 20.x), 🚫 Lint, Typecheck, Format, and Test (ubuntu-latest, 20.x), 🚫 Check for .changeset entries for all changed files (ubuntu-latest, 20.x), 🚫 Jest Coverage (ubuntu-latest, 20.x), 🚫 Check builds for changes in size (ubuntu-latest, 20.x), ✅ Publish Storybook to Chromatic (ubuntu-latest, 20.x), ✅ gerald

Pull Request URL: #1603

packages/perseus-editor/src/widgets/__tests__/interactive-graph-editor.test.tsx

@@ -574,10 +574,7 @@ describe("InteractiveGraphEditor", () => {
 
     test("getSaveWarnings returns an error when the graph is invalid", async () => {
         // Arrange
-        jest.spyOn(React, "useRef").mockReturnValue({
-            current: null,
-        });
-        const ref = React.useRef<InteractiveGraphEditor>(null);
+        const ref = React.createRef<InteractiveGraphEditor>();
 
         // Act
         render(
@@ -611,10 +608,7 @@ describe("InteractiveGraphEditor", () => {
 
     test("getSaveWarnings is empty if there are no errors", async () => {
         // Arrange
-        jest.spyOn(React, "useRef").mockReturnValue({
-            current: null,
-        });
-        const ref = React.useRef<InteractiveGraphEditor>(null);
+        const ref = React.createRef<InteractiveGraphEditor>();
 
         // Act
         render(