[ci]: Support PR coverage (sonic-net#1806)

The output log will be collected by subsequent pipelines to generate Pull Request coverage reports.
And if a branch wants to enforce the coverage rate, we can simply and "--fail-under SCORE" in the cover-diff command, see detail in https://diff-cover.readthedocs.io/en/latest/README.html

azure-pipelines.yml

@@ -91,6 +91,14 @@ stages:
         python3 setup.py test
       displayName: 'Test Python 3'
 
+    - script: |
+        sudo pip3 install diff-cover
+        target_branch=$(System.PullRequest.TargetBranch)
+        compare_branch=origin/${target_branch#refs/heads/}
+        diff-cover coverage.xml --compare-branch=$compare_branch
+      condition: eq(variables['Build.Reason'], 'PullRequest')
+      displayName: "Diff coverage"
+
     - task: PublishTestResults@2
       inputs:
         testResultsFiles: '$(System.DefaultWorkingDirectory)/test-results.xml'