Add warnings are errors option

[michaellevy]

From @jimhester here:

What you would likely want to do is modify https://github.com/r-lib/testthat/blob/master/R/reporter-check.R to additionally tally expectation_warning and then stop for warnings in a similar way to how it stops for errors.

But maybe at this point it would be better to open an issue in the testthat repository and we can consider adding a stop_for_warning option to the check reporter.

[jimhester]

As of #721 you can now set options(warn = 2) to change warnings into errors in your tests, which would then fail the build if desired. So I am going to close this for now.