Make the advice to write unit tests for OT more prominent

[past]

Is your feature request related to a problem? Please describe.
Feature authors frequently omit writing tests for the origin Trial integration with their feature, which ends up in launching trials that don't work.

Describe the solution you'd like
The docs provide guidance on automated and manual testing. We should make that advice more prominent in the trial creation flow.

• Add a bullet point to the trial creation interstitial with content like "Web tests have been written and pass"
  

• Add a validation checkbox in the ot_creation_request form with a text like "Web tests have been written and pass". Maybe even asking for a chromium file link to verify existence?