Add tests for PSK cipher suites.

Only the three plain PSK suites for now. ECDHE_PSK_WITH_AES_128_GCM_SHA256 will
be in a follow-up.

Change-Id: Iafc116a5b2798c61d90c139b461cf98897ae23b3
Reviewed-on: https://boringssl-review.googlesource.com/2051
Reviewed-by: Adam Langley <agl@google.com>