Changes

s1gw: S1GW_Tests.default: fix empty junit-xml-*.log files

I copied this line from some other testsuite (likely ttcn3-mme-test).
For some reason it makes TITAN produce empty junit-xml-*.log files,
which are essential for the Test Results Analyzer in Jenkins.

Change-Id: Ia80bf9640a465dd748f0a47c543b9a745a236133
Related: SYS#6772