Restructure source and tests

__tests__/index.test.js

@@ -0,0 +1,18 @@
+/**
+ * Unit tests for the action's entrypoint, src/index.ts
+ */
+
+const { run } = require('../src/main')
+
+// Mock the action's entrypoint
+jest.mock('../src/main', () => ({
+  run: jest.fn()
+}))
+
+describe('index', () => {
+  it('calls run when imported', async () => {
+    require('../src/index')
+
+    expect(run).toHaveBeenCalled()
+  })
+})