ci: ensure XML test reports do not clobber the output of previous runs

Change-Id: I5ad31d571d296b6cd85012786ab3d4370f98fbea
Refs: #3750

.jenkins.d/20-tests.sh

@@ -12,13 +12,12 @@ rm -Rf ~/.ndn
 BOOST_VERSION=$(python -c "import sys; sys.path.append('build/c4che'); import _cache; print(_cache.BOOST_VERSION_NUMBER);")
 
 ut_log_args() {
-    ((ut_count+=1))
     if (( BOOST_VERSION >= 106200 )); then
-        echo --logger=HRF,test_suite,stdout:XML,all,build/xunit-report-${ut_count}.xml
+        echo --logger=HRF,test_suite,stdout:XML,all,build/xunit-${1:-report}.xml
     else
         if [[ -n $XUNIT ]]; then
             echo --log_level=all $( (( BOOST_VERSION >= 106000 )) && echo -- ) \
-                 --log_format2=XML --log_sink2=build/xunit-report-${ut_count}.xml
+                 --log_format2=XML --log_sink2=build/xunit-${1:-report}.xml
         else
             echo --log_level=test_suite
         fi
@@ -38,5 +37,3 @@ export ASAN_OPTIONS
 
 # Run unit tests
 ./build/unit-tests $(ut_log_args)
-
-unset ut_count