Bug 840292: Change debugging server xpcshell tests to use a "listTabs" request, rather than a "listContexts" request. r=dcamp

The original protocol specification had a "listContexts" request, but that
was only ever implemented in the test harness. The real server has always
used a browser-appropriate "listTabs" request. The latter is now
documented, and the former has been removed from the spec.

This is preparation for using a common root actor implementation for all
servers: the xpcshell tests now actually use the protocol such a common
root actor would speak.