Remove the solution input to make those functions not defined and you can see the compile error message which contains the assert statements

To prevent is simple, just put the reference solution in the complete test instead of preload section
And change the test like this

auto expected = _f2(r);
Assert::That(fromLithuanianToAustrianGrade(r), Equals(expected));

You need to mention the format of showItems, aka every item's print() and a \n after that.

Looks good to me, reviewed and approved :)