Add factorial test

Author: iamvalentin23

src/_Problems_/factorial/index.test.js

@@ -0,0 +1,15 @@
+const { factorial } = require('.');
+
+describe('Factorial', () => {
+  it('Should return 24', () => {
+    expect(factorial(4)).toEqual(24);
+  });
+
+  it('Should return 1', () => {
+    expect(factorial(1)).toEqual(1);
+  });
+
+  it('Should return 120', () => {
+    expect(factorial(5)).toEqual(120);
+  });
+});