Add feature `assert_message` to `RunningContainer` available in the test body.

Implement a method to assert that a message has appeared in the log stream of a RunningContainer.

Example:

let mut test = DockerTest::new();
let composition = Composition::with_repository("dockertest-rs/hello");

test.add_composition(composition);

test.run(|ops| async move {
    let hello = ops.handle("dockertest-rs/hello");
    hello.assert_message("hello dockertest-rs", MessageSource::Stdout, 5).await;
});