Improve performance of TestFx

[vidai-msft]

Description

Work done in the PR:

• Improved performance for generating TestEnvironment object multiple times
  • Instantiated the TestEnvironment based on the testcredential.json file only once
  • For each test case, SubscriptionId will be updated based on the recorded json file
  • Updated TestEnvironmentFactory.cs and the TestFxEnvironment property in MockContext.cs
  • Updated ApiManagementHelper to invoke with the right environment object
• Removed null check for SubscriptionId in Playback mode
• Added back data plane service client for OperationalInsights

Checklist

• [x] Check this box to confirm: I have read the Submitting Changes section of CONTRIBUTING.md and reviewed the following information:

• SHOULD select appropriate branch. Cmdlets from Autorest.PowerShell should go to generation branch.
• SHOULD make the title of PR clear and informative, and in the present imperative tense.
• SHOULD update ChangeLog.md file(s) appropriately
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header in the past tense. Add changelog in description section if PR goes into generation branch.
  • Should not change ChangeLog.md if no new release is required, such as fixing test case only.
• SHOULD have approved design review for the changes in this repository (Microsoft internal only) with following situations
  • Create new module from the scratch
  • Create new resource types which are not easy to conform to Azure PowerShell Design Guidelines
  • Create new resource type which name doesn't use module name as prefix
  • Have design question before implementation
• SHOULD regenerate markdown help files if there is cmdlet API change. Instruction
• SHOULD have proper test coverage for changes in pull request.
• SHOULD NOT introduce breaking changes in Az minor release except preview version.
• SHOULD NOT change version of module in pull request