connection-test: Encode size in message headers correctly

In these tests, message sizes are inconsistently encoded in either the upper
or lower 16 bits of the second word of the message. Resolve this in favour
of using the upper 16 bits, as this is how messages are supposed to be
encoded, even though that aspect of message decoding isn't being tested
here.

Signed-off-by: Fergus Dall <sidereal@google.com>