github: add CI for pull requests

and hacks to get tests working

src/components/App.test.tsx

@@ -1,9 +0,0 @@
-import { render } from '@testing-library/react';
-import React from 'react';
-import App from './App';
-
-test('renders learn react link', () => {
-    const { getByText } = render(<App />);
-    const linkElement = getByText(/learn react/i);
-    expect(linkElement).toBeInTheDocument();
-});

src/utils/math.test.ts

@@ -0,0 +1,9 @@
+import { fmod } from './math';
+
+test('fmod of positive numbers', () => {
+    expect(fmod(3, 4)).toBe(3 % 4);
+});
+
+test('fmod of negative and positive number', () => {
+    expect(fmod(-3, 4)).toBe(1);
+});