Add unittest

Add unittest
[   19s] + ./include_dir_macros
[   19s]
[   19s] running 6 tests
[   19s] test tests::invalid_variables ... ok
[   19s] test tests::dont_resolve_recursively ... ok
[   19s] test tests::parse_valid_identifiers ... ok
[   19s] test tests::resolve_path_with_no_environment_variables ... ok
[   19s] test tests::simple_environment_variable ... ok
[   19s] test tests::unknown_environment_variable ... ok
[   19s]
[   19s] test result: ok. 6 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s