Testing custom `{knitr}` engines without gaining soft dependencies

[IndrajeetPatil]

In https://github.com/r-lib/lintr/pull/1552, we gained {bookdown} and {tufte} as soft dependencies. In future, we should try to find a way to test these engines without having to install these dependencies.

For a possible solution, see this comment from and the discussion following it.

For other relevant discussion, also see https://github.com/yihui/knitr/issues/2181.

[MichaelChirico]

I think at least we can recommend users to namespace-qualify packages if they want lintr to work properly. That should work pretty well for us.