Initial change to add cts tests to runtest.

Adds cts test definitions to the testrunner/test_defs.xml.
Adds support for runtest --cts arg, which will run all cts tests.
This temporarily relies on the addition of a 'cts' attribute to the test
definition - a new xml format may be defined later that changes how cts tests
are identified.

This change is based on previous unsubmitted CL
https://android-git.corp.google.com/g/Gerrit#change,1702. The only delta wrt
to that change is the use of InstrumentationCtsTestRunner.

Update: rebased to latest donut.

testrunner/errors.py

@@ -34,7 +34,7 @@ class AbortError(Exception):
   """Generic exception that indicates a fatal error has occurred and program
   execution should be aborted."""
 
-  def __init__(self, msg="AbortError"):
+  def __init__(self, msg=""):
     self.msg = msg