test.rb

require 'test/unit'
require 'rubygems'
require 'mocha'

class Foo
  def self.say_hello_to(name)
    Bar.hello(name)
  end
end

class Bar
end

class TestFooWithStubbingInSetup < Test::Unit::TestCase
  def setup
    @hello = mock("said hello")
    Bar.stubs(:hello).returns(@hello)
  end

  def test_something
    Bar.expects(:hello).with("zab")
    Foo.say_hello_to("baz")
  end

  def test_something_else
    assert_equal @hello, Foo.say_hello_to("bob")
  end

  # Lots of other tests so it's convenient to stub Bar.hello in the setup...
end

class TestFooWithoutStubbingInSetup < Test::Unit::TestCase
  def setup
    @hello = mock("said hello")
  end

  def test_something
    Bar.expects(:hello).with("zab")
    Foo.say_hello_to("baz")
  end

  def test_something_else
    Bar.stubs(:hello).returns(@hello)
    assert_equal @hello, Foo.say_hello_to("bob")
  end
end