Adding `testthat` to `Suggests` and `external.packages` snapshots dependencies

If I add the package testthat to both external.packages in packrat/packrat.opts and the Suggests-field in the DESCRIPTION the dependencies of testthat are included in the snapshot. This seems unnecessary as one could assume that the external package will take care of its own dependencies.

Steps to reproduce:

1. Create a plain new project in RStudio.
2. Enable packrat while excluding testthat
3. Add Suggests: testthat to DESCRIPTION
4. Run:

> packrat::snapshot()

Adding these packages to packrat:
             _       
    R6         2.2.2 
    crayon     1.3.2 
    digest     0.6.12
    magrittr   1.5   
    praise     1.0.0 

Note that the directory packrat/lib doesn't contain any installed packages except packrat itself, a subsequent packrat::restore does install the dependencies.

Start with the snapshot behavior involving packrat/packrat.opts and the Suggests field in DESCRIPTION. Reproduce the steps with testthat excluded, then compare the packages recorded by packrat::snapshot() with those installed by packrat::restore(). Done means external packages do not cause their own dependencies to be added unnecessarily to the snapshot.