Add section on testing R packages

[mfansler]

This fills out some guidance on testing R packages, with some specific directions on running testthat tests (a common testing framework).

Note that one could have a simpler looking recipe by installing the tests (add --install-tests to build command) and test with testthat::test_package('PackageName'). However, I've omitted that since I believe it preferable to avoid delivering the test files to end users unnecessarily. Instead, I provide only the boilerplate that gives the same functionality sans copying. This is personally what I've shifted toward, but I still have some older recipes that install the tests.

Happy to take suggestions!

PR Checklist:

• note any issues closed by this PR with closing keywords
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[mfansler]

@conda-forge/r feel free to adjust or make suggestions!

[mfansler]

@conda-forge/core ready for review

[jdblischak]

I think this is great advice. Especially when the test suite runs quickly, I think it makes sense to explicitly test the installed R package when building the recipe. Thanks @mfansler for writing up this documentation!

[mfansler]

@conda-forge/core nudge

[mfansler]

@conda-forge/core bump

[xhochy]

Can you rebase this? Then we should be able to merge.

[jaimergp]

For clarity, adding_pkgs is now under sphinx/src/maintainer/adding_pkgs/, but other than that it shouldn't have received too many updates. Just make sure the lists render properly in the Netlify preview (the Markdown renderer is a bit strict with indentation and preceding whitespace).

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[mfansler]

Rebased; rendered preview LGTM.