Code review comment for lp:~robertcarr/mir/implement-client-credentials

I'm seeing a test failure with this branch:

$ bin/unit-tests --gtest_filter=ProtobufCommunicator.double_disconnection_attempt_has_no_effect
Note: Google Test filter = ProtobufCommunicator.double_disconnection_attempt_has_no_effect
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from ProtobufCommunicator
[ RUN ] ProtobufCommunicator.double_disconnection_attempt_has_no_effect
/home/alan/display_server/trunk-mir/tests/unit-tests/frontend/test_protobuf_communicator.cpp:213: Failure
Mock function called more times than expected - taking default action specified at:
/home/alan/display_server/mir5/tests/mir_test_doubles/test_protobuf_client.cpp:55:
    Function call: disconnect_done()
         Expected: to be never called
           Actual: called once - over-saturated and active
[ FAILED ] ProtobufCommunicator.double_disconnection_attempt_has_no_effect (3 ms)
[----------] 1 test from ProtobufCommunicator (3 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (4 ms total)
[ PASSED ] 0 tests.
[ FAILED ] 1 test, listed below:
[ FAILED ] ProtobufCommunicator.double_disconnection_attempt_has_no_effect

 1 FAILED TEST