ssh: fix data race in the doClientServerAuth test helper

The helper returned the auth errors collected via AuthLogCallback
without waiting for the server goroutine that appends to them. Close
the client side of the connection on handshake failure, to unblock a
server still reading from it, and join the server goroutine before
returning.

Change-Id: Ibe8c3b236491769f3433f7457d4f2b9a07a65be0
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/797043
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: David Chase <drchase@google.com>