kbuild: rust_is_available: add test suite

The `rust_is_available.sh` script runs for everybody compiling the
kernel, even if not using Rust. Therefore, it is important to ensure
that the script is correct to avoid breaking people's compilation.

In addition, the script needs to be able to handle a set of subtle
cases, including parsing version strings of different tools.

Therefore, maintenance of this script can be greatly eased with
a set of tests.

Thus add a test suite to cover hopefully most of the setups that
the script may encounter in the wild. Extra setups can be easily
added later on if missing.

The script currently covers all the branches of the shell script,
including several ways in which they may be entered.

Python is used for this script, since the script under test
does not depend on Rust, thus hopefully making it easier for others
to use if the need arises.

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Link: https://lore.kernel.org/r/20230616001631.463536-12-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>