test: always perform assertions in module API test

This commit pursues several goals:

* Eliminate unused parameter/variable warnings at building module_api.c
  in non-debug configuration. The problem was introduced in commit
  5c1bc3da ("decimal: add the library into the module API").
* Eliminate a need to check newly added tests in two build
  configurations (Debug and RelWithDebInfo) and don't forget to add
  `(void)x;` statements in addition to a test condition check.
* Fail the testing if conditions required by the
  app-tap/module_api.test.lua test are not met -- not only in the Debug
  build, but also in RelWithDebInfo.

Fixes #7625

NO_DOC=a change in a test, purely development matter
NO_CHANGELOG=see NO_DOC