test(lua): add some comments to inductive datatype test

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(lua): add tests for exercising datatype validation code

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(lua): add test for inductive datatype positivity check

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>