Improved epub:switch test in document 0100

Reading systems may ignore content in unrecognised elements, thus
excluding the 'FAIL' text in the switch-010 test. Changing the
epub:case branch to use a <p> element instead means that only the
behaviour of processing the epub:switch statement is tested.
Merge pull request #50 from rhdunn/master

Improved epub:switch test in document 0100