Specify Simpletest groups that should not be tested. For instance if you want to run tests only on custom code then you might want to exclude core groups:
  Actions, Aggregator, AJAX, Batch API, Block, Blog, Book, Bootstrap, Cache, Color, Comment, Contact, Contextual, Dashboard, Database, DBLog, Entity API, Field API, Field types, Field UI, File, File API, File API (remote), Filter, Form API, Forum, Help, Image, Locale, Mail, Menu, Module, Node, OpenID, Pager, Path, Path API, PHP, Poll, Profile, RDF, Search, Session, Shortcut, SimpleTest, Statistics, Syslog, System, Taxonomy, Theme, Tracker, Translation, Trigger, Update, Update API, Upgrade path, User, XML-RPC
  
Multiple groups can be separated by a comma.