Enable samples for formrecognizer (Azure#37676)

Author: xiangyan99

sdk/formrecognizer/tests.yml

@@ -8,8 +8,7 @@ extends:
       TestTimeoutInMinutes: 200
       UseFederatedAuth: true
       MatrixReplace:
-        # Disable samples tests for now as some of them are haning.
-        # - TestSamples=.*/true
+        - TestSamples=.*/true
       Clouds: 'Prod'
       # This is a specific request from the formrecognizer service team
       # their claim is that the full matrix ends up stress-testing their service.
@@ -25,3 +24,6 @@ extends:
         TEST_MODE: 'RunLiveNoRecord'
         AZURE_SKIP_LIVE_RECORDING: 'True'
         AZURE_TEST_RUN_LIVE: 'true'
+        # Set fake authority host to ensure Managed Identity fail for Default Azure Credential
+        # so "execute samples" step correctly picks up Powershell credential.
+        AZURE_POD_IDENTITY_AUTHORITY_HOST: "FakeAuthorityHost"