Cleanup the further the testsuite by doing the following:

• Remove assert!(result.is_ok()) pattern, which gives less context in case of failure than result.unwrap()
• Move the example of the README directly inside the lib.rs to compile them. Fix some examples that silently didn't compile until then.
• Fix doc generation warning and add a ci check for it
• Detect slow test with cargo +nightly t --workspace -- -Z unstable-options --report-time --format json 2> /dev/null | jq and ignore them locally. This improves the developer experience.
• Update README to new test location since testscontainers migration

I split the PR into several commits to ease the review.