Disable C regression tests with C++ front-end when assert.h is used

[tautschnig]

MacOS 10.15 CI builds fail as those now include type_traits, which uses
features our C++ front-end does not yet support.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
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[martin-cs]

You might want to have a look at #5541

[tautschnig]

Closing in favour of #5541.