test: use google/go-cmp consistently when comparing APMEvent slices

Whenever we are comparing batches or event slices we should use go-cmp for nicer and more readable output.

See an example in https://github.com/elastic/apm-server/pull/10592

`assert.ElementsMatch` will print the whole slice, often complainig about extra elements without performing a diff of the two slices.
Use `cmp.Diff` so that we can see what is wrong and why the tests are failing