Add `rspectre` to CI

[dgollahon]

• This can help detect unused test setup on an ongoing basis and also helps me by giving me a bigger corpus of rspectre users.

[dgollahon]

Hmm, @mbj, it doesn't seem like it triggers the build. Maybe because I am modifying it as an external contributor?

[mbj]

@dgollahon no idea, going to invite you.

[mbj]

invited.