Reorganize test suite.

Reorganize test suite.
- Split testLoginLogout() into testBadLogin() and testLoginLogout().
- Mark testCreateSimpleArticle() as incomplete until the popup
  window opened by clicking on VIEW is handled correctly.
- Disable mod_rewrite so that the test suite does not depend on it.
- Remove superfluous check for Testing_Selenium class.
- Migrate TestConfiguration.php.dist to config.xml.dist.
  - Configure whitelist for code coverage information.
    - Be conservative for now and only add include/ and plugins/
      directories for now.
    - Set addUncoveredFilesFromWhitelist="false" because not all
      files are loadable (yet).
- Add documentation for code coverage reporting.