Add stop_on_failure argument to test_file

[vspinu]

My use case is the same as in #515. I want to be able to get the non-0 exit value from test_file.

It's a bit strange that test_dir, test_check and the internal test_files accept the stop_on_failure, but the workhorse test_file doesn't.

The simplest way to address this might be to export test_files.

[DDorch]

Exporting test_files could also be a way to solve the feature #916 :)

[jimhester]

I the meantime testthat::test_file("tests/testthat.R", reporter = c("summary", "fail")) will do this without any code changes to testthat.

[hadley]

It might make sense to export test_files() and then push people towards using that. test_files() really seems like the natural primitive here; test_file() is just a special case with one path.

Something about the whole structure of test-files.R and test-directory.R feels a bit off, and I think would benefit from refactoring.

[hadley]

Currently test_file() is half an internal helper that's used by test_dir() and half an equivalent of test_dir() for a single file. It does neither thing well: as an internal helper, it could assume that more setup is done in test_files(), and as a runner for a single file, it fails to run setup and teardown code.

[hadley]

It turns out that test_file() had one additional use — it was also used for testing. So it really had 3 uses, and was optimised for none of them.