Add tests and configure a CI pipeline

[pulsastrix]

In order to properly test changes to this crate without using real hardware, we should add a CI environment that allows running unit tests.

Additionally, we should add some tests (and make the examples we do have runnable as doctests).

Notes:

• libuci has a default for the directory UCI configuration files are stored in. In order to run tests, we either have to always call set_config_dir for every test or wrap all tests with a call to chroot.
• In order to run tests in a CI environment, it might suffice to either allow statically linking to libuci or to provide libuci in the CI environment. Setting up a full OpenWRT instance should not be necessary (I think).