tests using tune package

[topepo]

Some of these will take a while. Also, we may not want tune as a dependency since it takes a while to build (due to its dependencies).

Another option that we use a lot is to add the tests to the extratests repository. This is a faux R package that will never be submitted to CRAN., It is just used for long running tests and integration-type tests that check to make sure that cross-package parts work well. It runs on a daily cron job and runs the tests on CRAN package versions and the current devel versions.

I'd say, we we get close to a CRAN submission, maybe more the more expensive tests over there to ease the burden here.