Add a legacy stylesheet

Use the legacy stylesheet when Thematic is the active
theme and in legacy xhtml mode
Unit tests: move constant definitions to bootstrap file

Constants seem not to work when inside the
test-thematic class. Moving them to bootstrap.php
makes them globally accessible.

Global state is bad in unit tests, but this is needed to make the tests pass.
Unit tests: Add call to thematic_replace_loops() in setUp() method of test-legacy-xhtml.php

Replacing the loops need to be called explicitly.
Putting the call in the setUp() method makes the
ensures it is only needed once and not in every
test case