Fixed build machine issue since it doesn't use C# 7.

Author: MiYanni

src/ResourceManager/DataLakeAnalytics/Commands.DataLakeAnalytics.Test/ScenarioTests/AdlaTestsBase.cs

@@ -113,9 +113,10 @@ public void RunPsTestWorkflow(bool createWasbAccount,
                     ResourceGroupName = TestUtilities.GenerateName("abarg1");
                     TryCreateResourceGroup(ResourceGroupName, ResourceGroupLocation);
                     AzureBlobStoreName = TestUtilities.GenerateName("azureblob01");
+                    string storageSuffix;
                     AzureBlobStoreAccessKey = TryCreateStorageAccount(ResourceGroupName,
                         AzureBlobStoreName,
-                        "DataLakeAnalyticsTestStorage", "DataLakeAnalyticsTestStorageDescription", ResourceGroupLocation, out _);
+                        "DataLakeAnalyticsTestStorage", "DataLakeAnalyticsTestStorageDescription", ResourceGroupLocation, out storageSuffix);
                 }
 
                 try