mention resetting the test runner in the PR template

Author: Krastanov

.github/pull_request_template.md

@@ -2,4 +2,6 @@ Please make sure to leave this PR editable by maintainers of this repo.
 
 Please make sure you have consulted [CONTRIBUTING](https://github.com/JuliaGraphs/Graphs.jl/blob/master/CONTRIBUTING.md)
 
-The linter bot might add new commits to your PR (e.g. to take care of formatting issues). Feel free to overwrite these commits. 
+The linter bot might add new commits to your PR (e.g. to take care of formatting issues). Feel free to overwrite these commits. 
+
+If the linter bot is the last one to put in a commit, **THE TESTS WILL NOT RUN**. Either lint/format your contributions yourself as described in CONTRIBUTING.md or close&reopen the PR to reset the test runner.