Always enable API black box unit tests (#7696)

Always enable API black box unit tests (#7696)

Currently, when assertions are disabled, we do not enable any unit
tests. However, we have decided that it would be beneficial to do black
box unit testing of the API even when building cvc5 without assertions,
because the API is user facing. This commit makes the following changes:

- Always enables API black box unit tests
- Adds a test to check whether a buggy version of Clang is being used,
  which prevents the use of `-fno-access-control` for white box tests
- Fixes a spooky variable name in a Python unit test