Also, add support for the SSLKEYLOGFILE environment variable to the
tests, to simplify debugging of unexpected failures.

Updates #9671

Change-Id: I20a34a5824f083da93097b793d51e796d6eb302b
Reviewed-on: https://go-review.googlesource.com/c/147417
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>