[cv] make potentially flaky test fail fast.
[cv] make potentially flaky test fail fast.

Unfortunately, I can't repro, even while stressing out the CPU.
However, if it fails again on a builder, this CL should:
 * fail faster, not after 10 minutes
 * fail due its own assertion So(eg.Wait(), ShouldBeNil)
 * emit logs

Thus, I hope it'll be a bit easier to figure out what's the root cause.

R=tikuta

Bug: 1229873
Change-Id: Ida0fe983aab7faac0ae780df43f3e4311a276251
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3037541
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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