Update linting and testing process

__tests__/index.test.ts

@@ -0,0 +1,17 @@
+/**
+ * Unit tests for the action's entrypoint, src/index.ts
+ */
+
+import * as main from '../src/main'
+
+// Mock the action's entrypoint
+const runMock = jest.spyOn(main, 'run').mockImplementation()
+
+describe('index', () => {
+  it('calls run when imported', async () => {
+    // eslint-disable-next-line @typescript-eslint/no-require-imports
+    require('../src/index')
+
+    expect(runMock).toHaveBeenCalled()
+  })
+})