Tracking issue: Improvements to cargo-semver-checks' own CI and test system

[obi1kenobi]

• [x] split up the test suite into separate test crates highlighting each specific semver issue, while running all lints on all test crates: #203
• [ ] add a check that the Cargo.toml crate name in new/old matches the name of the directory where the new/old crates are located
• [ ] add a check that new/old crates have no cargo fmt issues
• [x] add a script for creating a new pair of test crates given the directory name (#236, #237)
• [x] add a script for creating a new lint file and corresponding test outputs file (because the error message when you forget to add the test outputs file is kind of obscure) (#236, #237)
• [ ] witness crates: #223
• [x] #235
• [ ] run cargo-semver-checks on the most popular crates on crates.io: #207
• [ ] generate rustdoc for the current version of the most popular crates on crates.io, then run cargo-semver-checks with that rustdoc as both baseline and current so that any reported issues are definitely false-positive