How does Ruby Test:Unit locate test classes in a deep nested folder?

How does Ruby Test:Unit locate test classes in a deep nested folder? I am a Java developer and I understand how JUnit and TestNG can scan folders with a defined filter to locate tests.

I am wondering if someone can explain to me how Ruby can locate test classes with a similar method, preferrably using some kind of filename filter??

Say, for example, I have a directory structure like so; then how could Ruby Test::Unit test runner locate only the tests ending with the letter 'b'??

project
  -main
  -tests
    -group1
      -test1a.rb
      -test1b.rb
    -group2
      -test2a.rb
      -test2b.rb
    -group3
      -test3a.rb
      -test3b.rb
    -group4
      -test4a.rb
      -test4b.rb

Links to documentation are helpful.