Reduce number of events of SiSPSeededTracksStandalone.py to try to avoid timeout of unit test in dbg build. (ATLASRECTS-3037)

Reduce number of events of SiSPSeededTracksStandalone.py to try to avoid timeout of unit test in dbg build.

http://atlas-computing.web.cern.ch/atlas-computing/links/distDirectory/gitwww/MASTERWebArea/nicos_web_areamaster_Athena_x86_64centos7gcc8dbg64BC7G8AthenaDbg/NICOS_TestLog_2019-06-04T2153/InnerDetector_InDetRecAlgs_SiSPSeededTrackFinder___SiSPSeededTrackFinderConf__SiSPSeededTrackFinderTest__m.html