devel/R-cran-mockr: New port: Mocking in R

• Submitter becomes maintainer

Provides a means to mock a package function, i.e., temporarily substitute
it for testing.
Designed as a drop-in replacement for the now deprecated
'testthat::with_mock()' and 'testthat::local_mock()'.

WWW: https://krlmlr.github.io/mockr/

Differential Revision: https://reviews.freebsd.org/D43735