Add ability to lint only test/benchmark/fuzz functions

[GiedriusS]

Thank you for this tool! I have a use case where I want to ban time.Sleep from test functions to avoid flaky tests. Instead of time.Sleep with a hard-coded timeout, I want to push others to use functions that would retry some operations continuously until some deadline. For that, faillint should be able to operate only on Test*, Benchmark*, and Fuzz* functions. What do you think? Would you be open to adding such an option?

[fatih]

Yeah I'm fine with it. Feel free open a PR.