Make CI run on `pull_request`

[osfanbuff63]

Purpose

Fixes #254

Approach

The test.yml had only:

name: Test
on: [push]

and it needs the pull_request check, so:

name: Test
on: [push, pull_request]

I've also included an if statement to skip the job on pull_request unless the source branch is on a fork.

Future work

Haven't been able to test, since to test it we'd have to merge this into master, fork the repo, and then make some change and make a PR. But not really sure if that's needed or not.