[cv][e2e] hide extreme verbosity behind a flag.
[cv][e2e] hide extreme verbosity behind a flag.

It's useful during desperate debugging stuck tests,
while the usual `go test -v` suffices in most cases.

R=qyearsley, yiwzhang

Change-Id: I0f354b627ee5f8dd827a7cfff20fb8959340d6a5
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2947252
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go