Register an action func to be called when a failpoint is evaluated

Feature Request

Is your feature request related to a problem? Please describe:
When writing a unit test, I want to know if a failpoint is evaluated and the evaluated value.

Describe the feature you'd like:
When enabling a failpoint, pass in a function callback func(val interface{}). Afterwards, whenever the failpoint is evaluated, failpoint is responsible to call callback.

Describe alternatives you've considered:
Allow user to register a function to replace term. Every time the failpoint is evaluated, it calls the function to get the value.

Teachability, Documentation, Adoption, Optimization:

The issue does not name files, tests, or entry points. Locate the failpoint enabling and evaluation paths, then determine how the callback API should receive each evaluated value and add coverage showing the callback runs whenever evaluation occurs.