Add a missing flushHandshake call to the TLS 1.3 handshake.

For when the PackHandshakeFlight tests get enabled.

Change-Id: Iee20fd27d88ed58f59af3b7e2dd92235d35af9ce
Reviewed-on: https://boringssl-review.googlesource.com/8663
Reviewed-by: David Benjamin <davidben@google.com>

ssl/test/runner/handshake_client.go

@@ -615,6 +615,7 @@ func (hs *clientHandshakeState) doTLS13Handshake() error {
 		finished.verifyData[0]++
 	}
 	c.writeRecord(recordTypeHandshake, finished.marshal())
+	c.flushHandshake()
 
 	// Switch to application data keys.
 	c.out.updateKeys(deriveTrafficAEAD(c.vers, hs.suite, trafficSecret, applicationPhase, clientWrite), c.vers)