unit testing and code coverage

Functions that start with Test could be considered as unit tests and they can return a boolean condition to indicate pass or fail:

```
FUNCTION Add(a, b)
    Add = a + b
END FUNCTION

FUNCTION TestAdd
    TestAdd = Add(1, 2) = 3
END FUNCTION
```

```
basic-interpreter-rust PROGRAM.BAS --test --with-code-coverage
```