internal/relui,task: expand testing coverage for releases

Among other things, we add testing for idempotency
behaviors in security coalescing, expected fail open
errors per task, RC branch coalescing, and disclosure
safety.

Change-Id: I6ccdfb066e451b61ed80e87d43b038a0f99911ed
Reviewed-on: https://go-review.googlesource.com/c/build/+/820471
Reviewed-by: Nicholas Husin <nsh@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Neal Patel <nealpatel@google.com>

Author: thatnealpatel