meta-selftest: provide a recipe for zvariant

meta-selftest: provide a recipe for zvariant

This recipe is for showing a "real world" example of
a crate that depends on some git repositories.

Usually, this kind of crate is built within a global
workspace (here it is the zbus project) and so
doesn't need a Cargo.lock on its own.

For the sake of the demonstration, I had to tweak things
a little to be able to compile zvariant in standalone
(no relative path in dependency, no symlink to LICENSE
provide a Cargo.lock)

The use case where the crate had some git repository
in dependency is very common for "private" crate that
are not aimed to be published on crates.io.
When the project grow bigger, it is common to have
a bin and multiple lib developped in parallel, and these
libs are surely on a git repostitory.

A test case have been also added to check for:
  - the previous patch about git subpath parameter and devtool
  - the correctness of overriding dependencies (first patch of the
series)

Signed-off-by: Frederic Martinsons <frederic.martinsons@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>