something.rb

# how to test that this works?
# or would you even bother?

...

begin
  require 'system_timer'
rescue LoadError
  require 'timeout'
  SystemTimer = Timeout
end

...

something_spec.rb

before(:all) do
  Object.remove_const(SystemTimer)
end

after(:each) do
  Object.remove_const(SystemTimer)
end

after(:all) do
  load "something"
end

it "should require 'system_timer'" do
  Object.expects(:require).once.with('system_timer')
  load "something"
end

it "should use Timer as SystemTimer if system_timer could not be loaded"
  def Object.require # you should alias the old one first so you can alias it back later.
    raise LoadError, "..."
  end

  load "something"
  Time.should eql(SystemTimer)
end

spec_helper.rb

$LOAD_PATH.unshift(File.dirname(__FILE__))
$LOAD_PATH.unshift(File.join(File.dirname(__FILE__), '..', 'lib'))

require 'rubygems'
require 'spec'
require 'webmock/rspec'
require 'something' # <--- it's already loaded here
require 'faker'