Add tests for ECDHE_PSK.

pskKeyAgreement is now a wrapper over a base key agreement.

Change-Id: Ic18862d3e98f7513476f878b8df5dcd8d36a0eac
Reviewed-on: https://boringssl-review.googlesource.com/2053
Reviewed-by: Adam Langley <agl@google.com>
3 files changed