tests: Add a docstring to fix CI

It looks like with Rust 1.83, `#[deny(missing_docs)]` started
triggering for our integration test. It looks like this is
a known thing: rust-lang/rust#130203 (comment)

Signed-off-by: Colin Walters <[email protected]>

Author: cgwalters

ostree-ext/tests/it/main.rs

@@ -1,3 +1,5 @@
+//! Main integration tests that use the public APIs.
+
 use anyhow::{Context, Result};
 use camino::Utf8Path;
 use cap_std::fs::{Dir, DirBuilder, DirBuilderExt};