crypto/tls: simplify keylog tests.

crypto/tls: simplify keylog tests.

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>