connection-test: Pad out strings with null bytes

The connection_demarshal test writes a 10 byte string into a wayland message,
but doesn't pad it out to a four byte boundary. This leads to the last 32-bit
word of the message being partially uninitialized, which triggers an msan
violation when the message is written to the socket.

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