Fix DebugContext test by ensuring that messages make it through the driver.

Bug: 18986756

There is a possible race condition between the error message being sent back
through the driver in the initial version of this code. By adding an explicit
message send/receive after that error message, we can ensure that the
expected error message was actually sent/received.

Change-Id: Ifbfee7526c930406f90cbf163f0e96bd896e6e96