Test both asynchronous and synchronous versions. This callback is somewhat
different from others. It's NOT called a second time when the handshake is
resumed. This appears to be intentional and not a mismerge from the internal
patch. The caller is expected to set up any state before resuming the handshake
state machine.
Also test the early callback returning an error.
Change-Id: If5e6eddd7007ea5cdd7533b4238e456106b95cbd
Reviewed-on: https://boringssl-review.googlesource.com/3590
Reviewed-by: Adam Langley <agl@google.com>
6f5c0f4471