failint: add ability to lint only tests

[GiedriusS]

Add the ability to lint only testing functions. This is useful to prevent hardcoded sleeps in tests leading to flaky tests.

I have never worked with the AST package in Go so this requires extra carefulness.

Solves https://github.com/fatih/faillint/issues/30.

[GiedriusS]

@fatih kind ping 🤗

[GiedriusS]

@fatih ping

[fatih]

@GiedriusS thank you. I opened a new PR that does it slightly different. You don't need to traverse the AST, hence it's better that way. Could you try it out? https://github.com/fatih/faillint/pull/43

[fatih]

Closing this in favor of https://github.com/fatih/faillint/pull/43