Add unit tests for StreamLayerClientImpl::GetData method

[mykolamalik]

Resolves: OLPEDGE-1456

Signed-off-by: Mykola Malik ext-mykola.malik@here.com

[codecov]

Codecov Report

Merging #603 into master will increase coverage by 0.3%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master    #603     +/-   ##
========================================
+ Coverage    76.8%   77.1%   +0.3%     
========================================
  Files         277     277             
  Lines        8675    8675             
========================================
+ Hits         6669    6697     +28     
+ Misses       2006    1978     -28

Impacted Files	Coverage Δ
...nclude/olp/dataservice/read/PrefetchTilesRequest.h	96.6% <0%> (ø)	⬆️
...d/include/olp/dataservice/read/PartitionsRequest.h	95.8% <0%> (ø)	⬆️
...ce-read/include/olp/dataservice/read/DataRequest.h	97.7% <0%> (ø)	⬆️
...read/include/olp/dataservice/read/model/Messages.h	100% <0%> (ø)	⬆️
.../include/olp/dataservice/read/ConsumerProperties.h	100% <0%> (ø)	⬆️
...read/include/olp/dataservice/read/CatalogRequest.h	87.5% <0%> (ø)	⬆️
...clude/olp/dataservice/read/CatalogVersionRequest.h	47.6% <0%> (ø)	⬆️
...ad/include/olp/dataservice/read/SubscribeRequest.h	100% <0%> (ø)	⬆️
...include/olp/dataservice/read/model/StreamOffsets.h	83.3% <0%> (ø)	⬆️
olp-cpp-sdk-core/src/cache/DefaultCache.cpp	90.6% <0%> (+1.7%)	⬆️
... and 2 more

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