Modify PR template to encourage contribution of automated tests

Many recent PRs offer up shell scripts or example queries in the testing
section, when these could be easily and usefully be converted to automated
tests based on MTR.  (https://github.com/MariaDB/server/pull/1888 and
https://github.com/MariaDB/server/pull/1940 are good examples of this.)

This modifies the PR template to more strongly encourage the contribution of
automated tests.

.github/pull_request_template.md

@@ -26,8 +26,16 @@ An amazing description should answer some questions like:
 TODO: fill description here
 
 ## How can this PR be tested?
-TODO: fill steps to reproduce here, if applicable,
-      or remove the section
+
+TODO: modify the automated test suite to verify that the PR causes MariaDB to
+behave as intended. Consult the documentation on
+["Writing good test cases"](https://mariadb.org/get-involved/getting-started-for-developers/writing-good-test-cases-mariadb-server).
+In many cases, this will be as simple as modifying one `.test` and one `.result`
+file in the `mysql-test/` subdirectory. Without _automated_ tests, future regressions
+in the expected behavior can't be automatically detected and verified.
+
+If the changes are not amenable to automated testing, please explain why not and
+carefully describe how to test manually.
 
 <!--
 Tick one of the following boxes [x] to help us understand
@@ -47,4 +55,3 @@ You might consider answering some questions like:
 -->
 ## Backward compatibility
 TODO: fill details here, if applicable, or remove the section
-