Warning when used in a test-only crate: ignoring invalid dependency

[dtolnay]

Cargo packages are not required to include a library target. Packages can be binaries-only, tests-only, examples-only, benches-only, or include multiple of those 4 things.

For example https://github.com/serde-rs/serde/tree/v1.0.147/test_suite is a tests-only Cargo package.

Currently trybuild's output displays a warning when used from a tests-only package or other package lacking a library target.

warning: repro-tests v0.0.0 (repro/target/tests/repro) ignoring invalid dependency `repro` which is missing a lib target