meta: Don't put the testing explanation in the PR text

By adding it as a comment, the user writing the PR will still see it but it won't be present in the rendered text.

.github/pull_request_template.md

@@ -1,9 +1,11 @@
-### Testing 
+<!--
+TESTING
+
 Writing tests is very important. Please try to write some tests for your PR. 
 If you need help, please do not hesitate to ask in this PR for help.
 
-[unit tests](https://github.com/nextcloud/android/blob/master/CONTRIBUTING.md#unit-tests)
-[instrumented tests](https://github.com/nextcloud/android/blob/master/CONTRIBUTING.md#instrumented-tests)
-[UI tests](https://github.com/nextcloud/android/blob/master/CONTRIBUTING.md#ui-tests)
-
+Unit tests: https://github.com/nextcloud/android/blob/master/CONTRIBUTING.md#unit-tests
+Instrumented tests: https://github.com/nextcloud/android/blob/master/CONTRIBUTING.md#instrumented-tests
+UI tests: https://github.com/nextcloud/android/blob/master/CONTRIBUTING.md#ui-tests
+ -->
 - [ ] Tests written, or not not needed