deep-exit: ignore testable examples

[alexandear]

I propose to ignore testable example functions in the deep-exit rule. There is nothing wrong with using log.Fatal or log.Fatalf in examples.

See these examples where using log.Fatal is normal:

• https://github.com/golang/go/blob/e5489a34ca2c31608821d3ac4ec07892fb6a2272/src/embed/example_test.go#L21
• https://github.com/google/go-github/blob/c6856bc9e29aeba0999a959f7a892098b18ff965/github/examples_test.go#L103
• https://github.com/golang/go/blob/e5489a34ca2c31608821d3ac4ec07892fb6a2272/src/image/decode_example_test.go#L28

[ccoVeille]

Thanks for this. I faced the exact same issue a few weeks ago