http2: add newly dialed conns to the pool before signaling completion
http2: add newly dialed conns to the pool before signaling completion

In dialCall.dial, close the done channel to mark dial completion after
adding the new connection to the clientConnPool.

Fixes a race condition in TestTransportBodyReadError, where the client
side of the test could observe the clientConnPool as unexpectedly
containing no conns, because the new conn had not yet been added to
the pool.

Fixes golang/go#44304.

Change-Id: I121200f9aa664fae29d0532e7fa2da47de3fe6a8
Reviewed-on: https://go-review.googlesource.com/c/net/+/410934
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>

golang/net