Run 20 vs tests at a time. (#1111)

In vs test, run 20 tests at a time instead of running all the tests together.
This may help in fixing the test issue.

.azure-pipelines/test-docker-sonic-vs-template.yml

@@ -64,7 +64,22 @@ jobs:
       docker ps
       ip netns list
       pushd sonic-swss/tests
-      sudo py.test -v --force-flaky --junitxml=tr.xml --imgname=docker-sonic-vs:$(Build.DefinitionName).$(Build.BuildNumber)
+      all_tests=$(ls test_*.py)
+      all_tests="${all_tests} p4rt"
+      test_set=()
+      # Run 20 tests as a set.
+      for test in ${all_tests}; do
+        test_set+=("${test}")
+        if [ ${#test_set[@]} -ge 20 ]; then
+          test_name=$(echo "${test_set[0]}" | cut -d "." -f 1)
+          echo "${test_set[*]}" | xargs sudo py.test -v --force-flaky --junitxml="${test_name}_tr.xml" --imgname=docker-sonic-vs:$(Build.DefinitionName).$(Build.BuildNumber)
+          test_set=()
+        fi
+      done
+      if [ ${#test_set[@]} -gt 0 ]; then
+        test_name=$(echo "${test_set[0]}" | cut -d "." -f 1)
+        echo "${test_set[*]}" | xargs sudo py.test -v --force-flaky --junitxml="${test_name}_tr.xml" --imgname=docker-sonic-vs:$(Build.DefinitionName).$(Build.BuildNumber)
+      fi
       rm -rf $(Build.ArtifactStagingDirectory)/download
     displayName: "Run vs tests"