Alertmanager: Create tests to prevent regressions in notifier requests

[stuartnelson3]

Issue is being created as a result of https://github.com/prometheus/alertmanager/pull/1516

My initial idea is to store the request, verify it's working / visually inspect it, and then have test requests be compared to that stored request. It's not perfect, but we'll be notified if we have changed the request being sent and can decide if we need to update the stored request or it's a regression.

Taken from https://www.youtube.com/watch?v=yszygk1cpEc&t=14m27s

Thoughts @simonpasquier @mxinden ?

[simonpasquier]

Another improvement would be to have notifier/XXX.go and notifier/XXX_test.go files for every notifier instead of the monolithic notifier/impl.go.