runner: Implement DHE-RSA.

Use it to test DHE-RSA in BoringSSL.

Change-Id: I88f7bfa76507a6f60234d61d494c9f94b7df4e0a
Reviewed-on: https://boringssl-review.googlesource.com/1377
Reviewed-by: Adam Langley <agl@google.com>