The synchronization in this test is a bit complicated and likely
incorrect, judging from the sporadically hanging trybots.
Most of what this is supposed to test is already tested in
TestTestContext, so I'll just remove it.

Fixes #15170

Change-Id: If54db977503caa109cec4516974eda9191051888
Reviewed-on: https://go-review.googlesource.com/22080
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>