[feature request] export on_cran

[pawelru]

As a developer
Sometimes I want to use exactly the same condition as in skip_on_cran().
An example could be: if not on cran - set following options in my testing setup file.

[hadley]

We don't want to export these functions because they are not directly related to the purpose of testthat. But you're welcome to copy and paste and recreate in your own package; they're generally pretty simple.