[az] Set diff coverage threshold to 50% (sonic-net#963)

Author: kcudnik

.azure-pipelines/build-template.yml

@@ -42,7 +42,7 @@ jobs:
   displayName: ${{ parameters.arch }}
   timeoutInMinutes: ${{ parameters.timeout }}
   variables:
-    DIFF_COVER_CHECK_THRESHOLD: 0
+    DIFF_COVER_CHECK_THRESHOLD: 50
     ${{ if eq(parameters.run_unit_test, true) }}:
       DIFF_COVER_ENABLE: 'true'