test: Fix payload and model opcode packing in test-mesh

test: Fix payload and model opcode packing in test-mesh

Use correct packing of multi-byte values in message payload bytearray.
For example, a 2-byte opcode 0x8204 is packed as 0x82 0x04, i.e. in
natural order.

Add transaction ID parameter to "set" commands of generic On/Off
model. Server will ignore the identical commands with the same
transaction ID, source and destination during a timeout period
of 6 seconds.

Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>