I put this at the end of my class files so that running the file directly runs the test for the file.

in_line_test_code.rb

class Someclass
  ...
end

########## inline test running
if __FILE__==$PROGRAM_NAME
  require_relative '../path_to_test.rb'
end

# This will run the relevant test if you run the class file. It will not run that code if the class file is just required from elsewhere, though.

#file: path_to_test.rb
# This part is also super easy for unit tests...

require 'test/unit'
class SomeclassTest < Test::Unit::TestCase
  def setup
    # setup code
  end
  def test_some_feature
    assert something_is_true, "it's not true!"
    assert_equal value_youre_expecting, Someclass.new.someemethod("params")
    # etc.
  end
end