Add a test helper for simple JSON serialisation testing

While adding yet another JSON serialisation test, I realised that this
can be trivially abstracted; hence this patch, replacing both simple
versions (readJSON . showJSON == id) and the standard version (with
different error messages) across the tests with a single function
call.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>