Add an empty `ctest` crate

A bit of a quirk of `release-plz` seems to be that it uses the latest
published stable Cargo.toml as the workspace root, which makes
publishing `ctest` a problem because i

Add an empty `ctest` crate

A bit of a quirk of `release-plz` seems to be that it uses the latest
published stable Cargo.toml as the workspace root, which makes
publishing `ctest` a problem because it doesn't exist (and so isn't in
the workspace) with our published `libc`, instead only existing on
`main`. Work around this by creating an empty crate named `ctest` so
the `libc-0.2` version of `Cargo.toml` lists it as a workspace member.

Link: https://github.com/release-plz/release-plz/issues/2163

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