Provide tests for Arg::default_value_if
Provide tests for Arg::default_value_if

While reading the code for the yaml translation, I've noticed that
there is a bug in the macro `yaml_opt_str` as well as a wrong `null`
value in the test fixture.

These tests add the expected behaviour on the given fixture, e.g.

    prog
and
    prog <WRONG>
where `<WRONG>` is **not** `other` should yield `None` for
`"positional2"`, whereas

    prog other
should yield "something" and

    prog --flag ARBITRARY_VALUE
should yield "some". The first two tests

- default_value_if_not_triggered
- default_value_if_not_triggered_by_argument

fail, as the second positional arguments *gets set*, although its
conditions aren't fulfilled.

clap-rs/clap,clap-rs/clap,clap-rs/clap