Add testing to review guidelines
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

vector-im/vector-web,vector-im/vector-web,vector-im/vector-web,vector-im/vector-web,vector-im/vector-web