github: add CI for pull requests

and hacks to get tests working

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);
+});