This test causes the server to send a GOAWAY and close a connection.
The server GOAWAY path writes a GOAWAY frame asynchronously, and
closes the connection if the write doesn't complete within 1s.
This is causing failures on some builders, when the frame write
doesn't complete in time.

The important aspect of this test is that the connection be closed.
Drop the check for the GOAWAY frame.

Change-Id: I099413be9c4dfe71d8fe83d2c6242e82e282293e
Reviewed-on: https://go-review.googlesource.com/c/net/+/576235
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>