Add support for test files in nested directories

[radbasa]

Closes #1605

Allows the use of directories to organize test files.

The recursive argument is optional and defaults to FALSE.

1. testthat::test_dir("tests/testthat/") will work as before. Just one single directory, and with the default reporter.
2. testthat::test_dir("tests/testthat/", recursive = TRUE) to run all of the test scripts in all directories in tests/testthat/. testthat::test_dir() will still only see test scripts with ^test.*\\.[rR]$ filenames.