CONTRIBUTING: Drop xmonad-testing

xmonad-testing isn't really being used, so the wording needs to be
changed in CONTRIBUTING.md and in the PR template. The important thing:
we want an example for new modules/functionality, and appropriate tests.

Related: https://github.com/xmonad/xmonad/issues/341

.github/PULL_REQUEST_TEMPLATE.md

@@ -9,6 +9,7 @@ behind them.
 
   - [ ] I've confirmed these changes don't belong in xmonad-contrib instead
 
-  - [ ] I tested my changes with [xmonad-testing](https://github.com/xmonad/xmonad-testing)
+  - [ ] I've considered how to best test these changes (property, unit,
+        manually, ...) and concluded: XXX
 
   - [ ] I updated the `CHANGES.md` file

CONTRIBUTING.md

@@ -61,9 +61,10 @@ Here are some tips for getting your changes merged into xmonad:
   * Your changes should include relevant entries in the `CHANGES.md`
     file.  Help us communicate changes to the community.
 
-  * Make sure you test your changes using the [xmonad-testing][]
-    repository.  Include a new configuration file that shows off your
-    changes if possible by creating a PR on that repository as well.
+  * Make sure you test your changes against the most recent commit of
+    [xmonad][] (and [xmonad-contrib][], if you're contributing there).
+    If you're adding a new module or functionality, make sure to add an
+    example in the documentation and in the PR description.
 
   * Make sure you run the automated tests.  Both [xmonad-contrib][]
     and [xmonad][] have test-suites that you could run with