Add testing to review guidelines

This adds a initial statement on testing to the review guidelines. No
requirements are made at this time, but reviewers are encouraged to request
tests where they believe it is reasonable to provide them.

Fixes https://github.com/vector-im/element-web/issues/14945

docs/review.md

@@ -14,6 +14,7 @@ When reviewing code, here are some things we look for and also things we avoid:
 * Performance
 * Accessibility
 * Security
+* Quality via automated and manual testing
 * Comments and documentation where needed
 * Sharing knowledge of different areas among the team
 * Ensuring it's something we're comfortable maintaining for the long term
@@ -34,6 +35,7 @@ When reviewing code, here are some things we look for and also things we avoid:
 * Reviewers should be explicit about required versus optional changes
   * Reviews are conversations and the PR author should feel comfortable
     discussing and pushing back on changes before making them
+* Reviewers are encouraged to ask for tests where they believe it is reasonable
 * Core team should lead by example through their tone and language
 * Take the time to thank and point out good code changes
 * Using softer language like "please" and "what do you think?" goes a long way