[web/test] don't delete uicache directory
Other tests use it, so the rmtree was a bit too much. Set CACHEDIR to a
temporary subdirectory, and only delete that.
option_group_changed('port', 'main', 'web')
option_group_changed('query-log-file', 'main', 'web')
option_group_changed('profile', 'main', 'web')
option_group_changed('max-post-length', 'main', 'web')