Simplify xpcshell ini files - Bug 894115 - xpcshell ini file confusion. r=bbondy
Part 1. use custom XREExeF and UpdRootD - Bug 889183 - mozapps/update xpcshell tests cannot be run concurrently. r=bbondy
Part 2. add run-sequentially to xpcshell ini files for tests that can't run in parallel yet - Bug 889183 - mozapps/update xpcshell tests cannot be run concurrently. r=mihneadb