MUnit testing a flow with collection-aggregator

I am trying to write MUnit tests for my Mule flows. I want to write a test for the below flow

The interesting bit is the exception sub-flow, especially the collection-aggregator

My unit test is

@Test
public void whenMultipleExceptionsOccurInFlow_itShouldSendOnlyOneFailureEmail() throws Exception {
    whenMessageProcessor("collection-aggregator")
            .withAttributes(attribute("name").ofNamespace("doc").withValue("Exception Aggregator")).thenReturnSameEvent();

    destinationSFTP.thenThrow(new RuntimeException("Dummy Exception destinationSFTP"));

    MuleEvent testEvent = PropertyEnricher.enrich(testEvent(IOUtils.toInputStream("hello,dummy,payload"))).get();

    runFlow("DownloadFTPFileIntoLocalFlow", testEvent);

    verifyCallOfMessageProcessor("outbound-endpoint").ofNamespace("smtp")
            .withAttributes(attribute("name").ofNamespace("doc").withValue("ErrorEmail"))
            .times(1);
}

Now if I do not mock the collection aggregator out my test does not pass, I can understand that this is tricky as the aggregator has a "pause" within it and hence is not an ideal candidate for a unit test, however from a technical standpoint I want to understand what is causing the unit test to fail (when collection-aggregator is not mocked).

The exception I get when the collection-aggregator is not mocked is as follows

    Message               : Dummy Exception destinationSFTP (java.lang.RuntimeException). Message payload is of type: String
Code                  : MULE_ERROR--2
--------------------------------------------------------------------------------
Exception stack is:
1. Dummy Exception destinationSFTP (java.lang.RuntimeException)
  nz.co.mightyriver.ProcessCsvTest:87 (null)
2. Dummy Exception destinationSFTP (java.lang.RuntimeException). Message payload is of type: String (org.mule.api.MessagingException)
  org.mule.execution.ExceptionToMessagingExceptionExecutionInterceptor:32 (http://ift.tt/1fnsxr9)
--------------------------------------------------------------------------------
Root Exception stack trace:
java.lang.RuntimeException: Dummy Exception destinationSFTP
    at nz.co.mightyriver.ProcessCsvTest.whenMultipleExceptionsOccurInFlow_itShouldSendOnlyOneFailureEmail(ProcessCsvTest.java:87)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    + 3 more (set debug level logging or '-Dmule.verbose.exceptions=true' for everything)

The exception does not pinpoint the root cause.