How to unit-test behavior under "version"?

[Andrey Zherikov via Digitalmars-d-learn]

I have a function below (just an example). What's the recommended 
way to unit-test it for all `version` cases?
```d
void f()
{
        import std.stdio: writeln;
    version(foo)
        writeln("foo");
    else
        writeln("no foo");
}
```