Issue in testcase with CAS Combiner

**Description**
Upon reviewing the [UIMA spec](https://uima.apache.org/d/uimaj-current/tug.html#ugr.tug.cm.cm_and_fc) for CAS Multiplier we realized the test case for CAS Combiner in [test_engine.cpp](https://github.com/apache/uima-uimacpp/blob/a710eb90b7eed3d17673bddb464cd29e6d654315/src/test/src/test_engine.cpp#L572) is wrong, and the input CASes to the Combiner should continue along the pipeline (the default behavior), instead of being dropped. This means that there should be 4 output CASes instead of 2.

**Fixes**
A simple fix could be to change the number of expected outputs and the related assertions. Further discussions may be required.