Add some basic CI/workflow

[ondrasouk]

It would be good to add some basic CI to run build and tests.

The build should be easy to add, but the tests are more problematic, since the problem in #1 can cause them to be flaky.

Also there is no reference picture for now, which is required by test ‎tests::clusters_test here https://github.com/ondrasouk/fast-slic-rust/blob/main/src/slic.rs#L423.