http2: use implicit synchronization in tests

Rewrite the synchronization used in Transport tests.
This replaces the explicit synchronization, which requires
annotating every point where a goroutine in the code under test
may block, with implicit syncronization based on parsing
goroutine stacks to identify when all goroutines of interest are
blocked.

Change-Id: I02646e2752c359ed1b08126370a48f3d3c1fde77
Reviewed-on: https://go-review.googlesource.com/c/net/+/584895
Reviewed-by: Jonathan Amsterdam <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: neild