Register an action func to be called when a failpoint is evaluated #64

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: