Add a section on the testing procedure for a pull request.
It should help reviewing pull requests as they often not describe which
application raises the error they are fixing or any way of verifying
that a feature is correctly implemented.

GitHub supports different types of issue templates for a while now.
The following types are supported:

* Bug report
  (living in `.github/ISSUE_TEMPLATE/bug_report.md`),
* Feature request
  (living in `.github/ISSUE_TEMPLATE/feature_request.md`), and
* Custom issue template
  (living in `.github/ISSUE_TEMPLATE/custom.md`)

Our current issue template is mostly a bug report template, so this
moves it to `.github/ISSUE_TEMPLATE/bug_report.md`.