[BEAM-12548] Add examples of Equals and EqualsList to equals_test.go (#15112)
[BEAM-12548] Add examples of Equals and EqualsList to equals_test.go (#15112)

* [BEAM-12548] Add examples of Equals and EqualsList to equals_test.go
* [BEAM-12548] Update EqualsList failure example to use a helper for unwrapping the error
* [BEAM-12548] Update helper and example to follow better go practices
lukecwik/incubator-beam,apache/beam,lukecwik/incubator-beam,lukecwik/incubator-beam,robertwb/incubator-beam,chamikaramj/beam,chamikaramj/beam,lukecwik/incubator-beam,robertwb/incubator-beam,robertwb/incubator-beam,lukecwik/incubator-beam,robertwb/incubator-beam,chamikaramj/beam,chamikaramj/beam,robertwb/incubator-beam,apache/beam,lukecwik/incubator-beam,apache/beam,robertwb/incubator-beam,apache/beam,robertwb/incubator-beam,lukecwik/incubator-beam,chamikaramj/beam,chamikaramj/beam,chamikaramj/beam,robertwb/incubator-beam,lukecwik/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,lukecwik/incubator-beam,apache/beam,robertwb/incubator-beam,apache/beam,apache/beam,apache/beam,chamikaramj/beam,apache/beam,apache/beam,chamikaramj/beam,robertwb/incubator-beam,apache/beam