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

[kruskall]

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

See an example in #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