Add DTLS timeout and retransmit tests.

This extends the packet adaptor protocol to send three commands:
  type command =
    | Packet of []byte
    | Timeout of time.Duration
    | TimeoutAck

When the shim processes a Timeout in BIO_read, it sends TimeoutAck, fails the
BIO_read, returns out of the SSL stack, advances the clock, calls
DTLSv1_handle_timeout, and continues.

If the Go side sends Timeout right between sending handshake flight N and
reading flight N+1, the shim won't read the Timeout until it has sent flight
N+1 (it only processes packet commands in BIO_read), so the TimeoutAck comes
after N+1. Go then drops all packets before the TimeoutAck, thus dropping one
transmit of flight N+1 without having to actually process the packets to
determine the end of the flight. The shim then sees the updated clock, calls
DTLSv1_handle_timeout, and re-sends flight N+1 for Go to process for real.

When dropping packets, Go checks the epoch and increments sequence numbers so
that we can continue to be strict here. This requires tracking the initial
sequence number of the next epoch.

The final Finished message takes an additional special-case to test. DTLS
triggers retransmits on either a timeout or seeing a stale flight. OpenSSL only
implements the former which should be sufficient (and is necessary) EXCEPT for
the final Finished message. If the peer's final Finished message is lost, it
won't be waiting for a message from us, so it won't time out anything. That
retransmit must be triggered on stale message, so we retransmit the Finished
message in Go.

Change-Id: I3ffbdb1de525beb2ee831b304670a3387877634c
Reviewed-on: https://boringssl-review.googlesource.com/3212
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/test/runner/common.go b/ssl/test/runner/common.go
index 7aaf9a2..99b0c04 100644
--- a/ssl/test/runner/common.go
+++ b/ssl/test/runner/common.go
@@ -603,6 +603,13 @@
 	// AppDataAfterChangeCipherSpec, if not null, causes application data to
 	// be sent immediately after ChangeCipherSpec.
 	AppDataAfterChangeCipherSpec []byte
+
+	// TimeoutSchedule is the schedule of packet drops and simulated
+	// timeouts for before each handshake leg from the peer.
+	TimeoutSchedule []time.Duration
+
+	// PacketAdaptor is the packetAdaptor to use to simulate timeouts.
+	PacketAdaptor *packetAdaptor
 }
 
 func (c *Config) serverInit() {