Gh-3108: Improve testing for GafferPopVertexProperty #1897

Job	Run time
Tests (ubuntu-latest, Core, :gaffer2,:core,:access,:cache,:common-util,:data,:exception,:graph,:o...	3m 52s
Build Javadoc	3m 20s
Tests (ubuntu-latest, Accumulo, :accumulo-store,:accumulo-rest)	29m 19s
Tests (ubuntu-latest, Federated-And-Map, :integration-test,:federated-store,:map-store,:map-rest)	17m 34s
Tests (ubuntu-latest, REST, :rest-api,:common-rest,:spring-rest,:core-rest,:store-implementation,...	5m 29s
Check all modules are tested	7s
Tests (ubuntu-latest, Examples, :example,:basic,:basic-model,:basic-rest,:road-traffic,:road-traf...	3m 36s
Tests (ubuntu-latest, Big-Data-Libraries, :flink-library,:hdfs-library,:spark,:spark-library,:spa...	8m 39s
Tests (ubuntu-latest, Time-Library, :library,:time-library)	1m 19s
Tests (ubuntu-latest, Caches, :cache-library,:sketches-library,:bitmap-library,:hazelcast-cache-s...	2m 21s
Tests (ubuntu-latest, Tinkerpop, :tinkerpop)	3m 3s
Upload coverage to Codecov	11s
	1h 18m 50s

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