[ci] pipeline fixes for VS test (#581)

*Cherry pick PRs sonic-net/sonic-swss#2142, sonic-net/sonic-swss#2138

Author: prsunny

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

@@ -11,10 +11,13 @@ jobs:
   displayName: vstest
   timeoutInMinutes: ${{ parameters.timeout }}
 
-  pool:
-    vmImage: 'ubuntu-20.04'
+  pool: sonic-common
 
   steps:
+  - script: |
+      ls -A1 | xargs -I{} sudo rm -rf {}
+    displayName: "Clean workspace"
+  - checkout: self
   - task: DownloadPipelineArtifact@2
     inputs:
       artifact: docker-sonic-vs