Give test_dir() a recursive option

[radbasa]

Because of this Appsilon/rhino#440, the recursive rhino::test_r() PR Appsilon/rhino#433 may not be sustainable. It works outside of testthat. To provide an experience similar to testthat::test_dir(), I just copied the stdout prompts of ProgressReporter. This cannot accommodate a specific reporter like JunitReporter or any other reporters.

This is a proposed solution inside testthat.

How to test

Build and install on local. Set up an R project with nested test directories.

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 only see scripts with the ^test.*\\.[rR]$ filename.
3. testthat::test_dir("tests/testthat/", recursive = TRUE, reporter = "Junit") the same as (2) above, but display Junit format.

Unit tests

1. testthat::test_dir("tests/testthat/") will show test-files has no failures.
2. testthat::test_file("tests/testthat/test-test-files.R") should pass.
3. devtools::test() throws an error even on main branch: Error in teardown_env()`.