Add testing to feature proposal template

What does this MR do?

Adds a testing section to the feature proposal template.

This is part of implementing a more lightweight test planning process (compared to always having a separate test plan issue) as outlined in https://gitlab.com/gitlab-org/quality/team-tasks/issues/88

It is intended to encourage everyone to be involved in test planning as early as possible, without significantly increasing anyone's workload beyond what it necessary to ensure the quality of our work.

It should be merged after (or with) gitlab-com/www-gitlab-com!20827 (merged).

Related issues

https://gitlab.com/gitlab-org/quality/team-tasks/issues/88 https://gitlab.com/gitlab-org/quality/team-tasks/issues/138