Stub all SoloCook commands to make option testing easier

Also use `in_kitchen` helper in `#test_no_librarian_option` to avoid
at least some of the stubs.
Unify librarian tests

Test all via `SoloCook#run` and by inspecting
`Librarian::Action::Install` invocations.
Add unit tests for Chef version checks
Add unit test for `--sync-only`
Merge branch 'cook-unit-tests'