PeerScoringReporterService is not properly released in unit tests

[Vovchyk]

Sometimes while running unit tests the following error appears:

com.fasterxml.jackson.databind.JsonMappingException: [no message for java.lang.NullPointerException]
        at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:303)
        at com.fasterxml.jackson.databind.ObjectMapper._configAndWriteValue(ObjectMapper.java:3681)
        at com.fasterxml.jackson.databind.ObjectMapper.writeValueAsString(ObjectMapper.java:3057)
        at co.rsk.scoring.PeerScoringReporterUtil.detailedReputationString(PeerScoringReporterUtil.java:50)
        at co.rsk.scoring.PeerScoringReporterService.printReports(PeerScoringReporterService.java:62)
        at co.rsk.scoring.PeerScoringReporterService.lambda$start$0(PeerScoringReporterService.java:38)
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
        at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
        at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException: null
        at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:124)
        at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:112)
        at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:25)
        at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:292)
        ... 12 common frames omitted

Seems that the PeerScoringReporterService is not properly disposed in the tests.

Note: this console output doesn't prevent the tests to succeed though.

Note 2: this does not happen on each run - appears on occasional basis