Reduce the number of events and the number of threads of TestSCTDecode.

Reduce the number of events from 10 to 6 and the number of threads from 5 to 3 to avoid timeout of TestSCTDecode.

http://atlas-computing.web.cern.ch/atlas-computing/links/distDirectory/gitwww/MASTERWebArea/nicos_web_areamaster_Athena_x86_64centos7gcc8opt64BC7G8AthenaOpt/NICOS_TestLog_2019-05-11T2132/InnerDetector_InDetEventCnv_SCT_RawDataByteStreamCnv___SCT_RawDataByteStreamCnvConf__SCT_RawDataByteStreamCnvTest__m.html