Cleanup and reorganize tests

First step is changing drupalGetSitemap():
1. To use contexts instead of the hard-coded $language parameter.
2. Remove the $regenerate parameter in favor of calling $this->regenerateSitemap() manually.
3. Remove the check for if response was 200 and be more like drupalGet() and not care about the response. Now that we're checking any watchdog messages in tearDown() we'll find any responses that 404'd automatically.

First round of cleaning up (#1) committed:
http://drupal.org/cvs?commit=364454
http://drupal.org/cvs?commit=364456
http://drupal.org/cvs?commit=364460
http://drupal.org/cvs?commit=364462

Next step: Adding an xmlsitemap_test.module to actually test the context system and adding/editing/deleting sitemaps.