Add system level tests for exe

[cmannett85]

The unit tests do not cover any code in the main target, for obvious reasons. This means we need system level tests to be added to the code base, which are ran by the pr_checker action. This is how #165 has escaped our attention.

An initial obvious approach is to provide a python script that execute the program with a known set of inputs (test/programs/*) and test the output against the expected.

A better alternative would be to change the main() signature if doing a test build (should change the unit_test.cmake file in this case...), expose it publicly by a test-build-only header, and then call it from a normal test case whilst hijacking std::cin and std::cout (or providing alternatives to it?) .