Add on_failure callback

[martin-schulze-vireso]

Is your feature request related to a problem? Please describe.
Sometimes we want to react to test failures in a specific way, doing special cleanup, inspecting in an interactive shell, ... currently, we can only use teardown and check whether the test completed.

Describe the solution you'd like
An on_failure callback function that gets called immediately, when the test is known to have failed. As a bonus this might allow for overriding the failure.

Describe alternatives you've considered
Special "failure only" cleanup can be done in teardown but is a bit more cumbersome. Dropping into an interactive shell could be solved with a specialized flag.

[wainersm]

Hi @martin-schulze-vireso ! As we talked in Gitter today, on kata-containers project we ran into a situation where this feature will be very useful.

As for """As a bonus this might allow for overriding the failure""", this is another feature very welcomed. Sometimes we know tests are failing and we want to keep them running instead of skipping, but we don't want the CI pipelines failing.