Move tests next to implementation, and fix filename (#1393)

## Summary:
The tests for axis ticks looked lonely hanging out in `__tests__`, so I
moved them next to the associated production code. I also renamed one
file from `tests.ts` to `test.ts` - the name was preventing Jest from
running the tests in that file.

Issue: none

## Test plan:

`yarn test`

Author: benchristel

Reviewers: mark-fitzgerald, SonicScrewdriver

Required Reviewers:

Approved By: mark-fitzgerald

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), ✅ Cypress (ubuntu-latest, 20.x), ✅ Jest Coverage (ubuntu-latest, 20.x), ✅ Check builds for changes in size (ubuntu-latest, 20.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ✅ Publish Storybook to Chromatic (ubuntu-latest, 20.x), ✅ gerald

Pull Request URL: #1393

Author: benchristel

.changeset/ten-paws-wash.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/perseus": patch
+---
+
+Internal: Move and rename test files

packages/perseus/src/widgets/interactive-graphs/__tests__/axis-tick-labels.tests.ts renamed to packages/perseus/src/widgets/interactive-graphs/axis-tick-labels.test.ts

@@ -1,4 +1,4 @@
-import {showTickLabel} from "../graphs/components/axis-tick-labels";
+import {showTickLabel} from "./graphs/components/axis-tick-labels";
 
 it("should hide the first negative axis tick label if the gridStep > tickStep", () => {
     const gridStep = 2;

packages/perseus/src/widgets/interactive-graphs/__tests__/axis-ticks.test.ts renamed to packages/perseus/src/widgets/interactive-graphs/axis-ticks.test.ts

@@ -1,4 +1,4 @@
-import {generateTickLocations} from "../axis-ticks";
+import {generateTickLocations} from "./axis-ticks";
 
 describe("generateTickLocations", () => {
     it("should generate ticks from the origin", () => {