Allow running Diagnostics on each request

[Thinkscape]

Use case (via Maxime Corbeau):

• The diagnostics would be run on any requests.
• IF all the tests are successful, then the request would be processed as usual.
• IF some of the tests fail, the processing of the request would be stopped and the diagnostics page would be returned.

The reason for wanting this is that the test environment can change frequently: the above setup would allow detecting changes immediately as they occur without having to manually check the diagnostics page (which is cumbersome, and doesn't guarantee detecting changes as they occur anyway because it's checked sporadically whereas the only way to do so it to check on any request).
This obviously would be disabled for production environment.