Add a simple unittest for THH code

This is very THH specific, and applies to all serialisations generated
by THH, so I'm adding it in its own module.

Probably we should add some more generic tests, but in general THH
code is tested by the various definitions; this new field type however
is not (yet), so this is why I want this specific unittest.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>