Like other language specific modules this module is module for holding
rust specific helpers. This commit adds a test() function, which
simplifies using rust's internal unittest mechanism.
Rust tests are generally placed in the same code files as they are
testing, in contrast to languages like C/C++ and python which generally
place the tests in separate translation units. For meson this is
somewhat problematic from a repetition point of view, as the only
changes are generally adding --test, and possibly some dependencies.
The rustmod.test() method provides a mechanism to remove the repatition:
it takes a rust target, copies it, and then addes the `--test` option,
then creates a Test() target with the `rust` protocol. You can pass
additional dependencies via the `dependencies` keyword. This all makes
for a nice, DRY, test definition.
This allows us to more easily have the documentation in sync with
the source code as people will have to document new features etc
right at the time where they implement it.
