As OpParams definitions are used at Template Haskell type, they don't
get any coverage (although the values defined by them are actually
used in unittests). However, we can at least test some of the
functions defined in the module.

This patch tests the failure modes of a few of the custom types in
this module.

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