[Solution] Project euler challenge 19 with tests (#1659)

* [Solution] Project euler challenge 19 with tests

* update leap year function

* Remove unnecessary, confusingly placed comments

---------

Co-authored-by: Lars Müller <34514239+appgurueu@users.noreply.github.com>

Project-Euler/test/Problem019.test.js

@@ -0,0 +1,8 @@
+import { problem19 } from '../Problem019.js'
+
+describe('checking sundays during the twentieth century', () => {
+  // Project Euler Challenge Check
+  test('result should be 171', () => {
+    expect(problem19()).toBe(171)
+  })
+})