[Test] Update test authoring guidelines (#1199)
[Test] Update test authoring guidelines (#1199)

Update guidelines for VCR test authoring.
yugangw-msft/azure-cli,yugangw-msft/azure-cli,QingChenmsft/azure-cli,yugangw-msft/azure-cli,samedder/azure-cli,samedder/azure-cli,samedder/azure-cli,yugangw-msft/azure-cli,QingChenmsft/azure-cli,QingChenmsft/azure-cli,yugangw-msft/azure-cli,yugangw-msft/azure-cli,QingChenmsft/azure-cli,samedder/azure-cli