Consider using Cargo's testing framework to implement offline validation tests

[PaulDance]

Cargo has its own test support framework: https://github.com/rust-lang/cargo/blob/master/crates/cargo-test-support/. An example of usage: https://github.com/hi-rustin/cargo-information/blob/main/tests/testsuite/cargo_information/basic/mod.rs. It should be a good candidate to implement validation tests that actually run in GitHub's CI that does not allow external connections.

trycmd or snapbox should also help in this endeavor.

Try terminal resizing hacks to check that the README.md tests work while still preserving a readable presentation. 70 columns should do fine.

[PaulDance]

Released as part of v0.5.0.