Test our CI tests

As discovered in https://github.com/google/zerocopy/commit/a8539dc2ea29d5c4dcd5b751ca072ca15b43ccce (credit to @tommy-gilligan), we have had bugs in our CI tests that resulted in them silently succeeding even when they should fail. We should test our CI tests. In particular, we should test the tests that are script-heavy. Many of them should be relatively straightforward to test simply by modifying a file such as `Cargo.toml` to have the wrong contents, and then calling one of the scripts in `ci/...` and asserting that it returns a non-zero status code.