Introduce fuzz testing

[kruskall]

Followup from #63

We should introduce fuzz testing to make sure we are not missing anything

As this is a library we might also evaluate adding fuzz testing into APM Server and fuzz the intake endpoint directly.

[kruskall]

RE #63 (comment):

the stdlib does not support structured fuzzing or fuzzing complex types (structs). We need a compatibility layer to cover that usage. In that PR I used https://github.com/google/gofuzz.

Gofuzz has full support for the stdlib fuzzing engine and the test looks like this:

func FuzzError(f *testing.F) {
	f.Fuzz(func(t *testing.T, data []byte) {
		c := complexStruct{}
		fuzz.NewFromGoFuzz(data).NilChance(.1).NumElements(1, 10).MaxDepth(5).Fuzz(&c) // populate complex struct
		foo(&c) // run target method with fuzzed data 
	})
}

The stdlib is generating random data ([]byte) which are used as a seed to populate the complex struct (this is to make the tests reproducible).

I looked into https://github.com/AdaLogics/go-fuzz-headers but my understanding is that it is used for https://github.com/dvyukov/go-fuzz and not to wire complex structs with the stdlib engine.

[axw]

https://adalogics.com/blog/structure-aware-go-fuzzing-complex-types gives some examples of using go-fuzz-headers with Go's native fuzz testing.