Since there's no aspect of key logging that OpenSSL can check for us,
the tests for it might as well just connect to another goroutine as this
is lower-maintainance.

Change-Id: I746d1dbad1b4bbfc8ef6ccf136ee4824dbda021e
Reviewed-on: https://go-review.googlesource.com/30089
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joonas Kuorilehto <joneskoo@derbian.fi>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>