Add the test plan to the PR template

Author: brettcannon

.github/PULL_REQUEST_TEMPLATE.md

@@ -8,4 +8,5 @@ For #
 - [ ] Title summarizes what is changing
 - [ ] Has a [news entry](https://github.com/Microsoft/vscode-python/tree/master/news) file (remember to thank yourself!)
 - [ ] Unit tests & system/integration tests are added/updated
+- [ ] [Test plan](https://github.com/Microsoft/vscode-python/blob/master/.github/test_plan.md) is updated as appropriate
 - [ ] [`package-lock.json`](https://github.com/Microsoft/vscode-python/blob/master/package-lock.json) has been regenerated by running `npm install` (if dependencies have changed)