SSDM-713: Refactoring screening system tests by moving common set ups into super class.

...creening/systemtests/AbstractScreeningSystemTestCase.java

...s/screening/systemtests/AggregatedFeatureVectorsTest.java

...tests/AggregationServiceAggregatedFeatureVectorsTest.java

...nbis/screening/systemtests/FeatureVectorsDropboxTest.java

...penbis/screening/systemtests/ImageBase64EncodingTest.java

...ng/systemtests/MiscellaneousDropBoxApiV2FeaturesTest.java

...reening/systemtests/ScreeningServerAuthorizationTest.java

...ning/systemtests/TransformedImageRepresentationsTest.java