Add `Kennel.in`; test tidying

[zdrve]

• Add Kennel.in
• In capture_all, set up Kennel.in to something consistent (so we don't rely on the test execution environment)
• Use Kennel.in/out/err consistently (never use STD... nor $std...)
• Give reset_instance a better name
• Remove redundant Kennel.@instance reset

Checklist

• [ ] Verified against local install of kennel (using path: in Gemfile)
• [ ] Added tests
• [ ] Updated Readme.md (if user facing behavior changed)