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.

tests/coverage/prepend.php

@@ -0,0 +1,9 @@
+<?php
+if ( isset($_COOKIE['PHPUNIT_SELENIUM_TEST_ID']) &&
+    !isset($_GET['PHPUNIT_SELENIUM_TEST_ID']) &&
+    extension_loaded('xdebug')) {
+    $GLOBALS['PHPUNIT_FILTERED_FILES'] = array(__FILE__);
+
+    xdebug_start_code_coverage(XDEBUG_CC_UNUSED | XDEBUG_CC_DEAD_CODE);
+}
+?>