CM_Page_ExampleTest doesn't clean up properly after itself

As reported by @mariansollmann 
`CM_Response_Resource_Css_LibraryTest` fails when running in combination with `CM_Page_ExampleTest`:
```
$ bin/phpunit --filter "/^CM_(Page|Response)/"
PHPUnit 4.1.6 by Sebastian Bergmann.

Configuration read from /home/vagrant/cm/phpunit.xml

.........S........FF....................

Time: 12.03 seconds, Memory: 76.00Mb

There were 2 failures:

1) CM_Response_Resource_Css_LibraryTest::testProcess
Failed asserting that 'html {
  font-family: sans-serif;
[...]
' contains "body{".
[...]
```