DefaultDrySiteTestCase only sets dry = True and provides no additional functionality. This is redundant since DefaultSiteTestCase already supports the dry class attribute. Remove DefaultDrySiteTestCase and update the DefaultSiteTestCase documentation to mention that dry = True can be used to run tests in offline mode.

Update tests accordingly: https://codesearch.wmcloud.org/pywikibot/?q=DefaultDrySiteTestCase&files=&excludeFiles=&repos=