Fix runner.go session ticket support.

Some test code to insert a bogus session ticket was retained. Also,
decryptTicket mutated its input, in turn, mutating the ClientHello,
breaking the Finished hash.

The latter fix should probably be merged upstream.

Change-Id: I6949f842c67e19df8742561fb0b849af9f5f099d
Reviewed-on: https://boringssl-review.googlesource.com/1290
Reviewed-by: Adam Langley <agl@google.com>