Allow code to output to console

[gergness]

In our CI tool, we can monitor logs interactively. However, covr::test_package() only allows running code in background processes that do not report to the console (the code provided in code in run via R CMD BATCH).

I wish there was some way to specify to show the output live in the console. I don't know what the API should look like, but will post a minimal PR of something that would work well for me.