internal/task: integration test for x repo tagging

Add an integration test for the x repo tagging workflow. Most of the
work is test infrastructure. Move the fake buildlet types into the task
package, and create a new fake Gerrit implementation that tracks tags
and commits. Also use it in the older release integration tests.

The coverage of the x repo tagging process itself is a bit thin but this
is a good starting point.

For golang/go#48523.

Change-Id: Iabf9391b56f6becadd401b8ae37a9e4b8cc5d24f
Reviewed-on: https://go-review.googlesource.com/c/build/+/425495
Auto-Submit: Heschi Kreinick <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Run-TryBot: Heschi Kreinick <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: heschi