`azuredevops_feed`, `azuredevops_feed_permission` - Optimize code and update test case

[xuzhang3]

All Submissions:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.
• [x] My code follows the code style of this project.
• [x] I ran lint checks locally prior to submission.
• [x] Have you checked to ensure there aren't other open PRs for the same update/change?

What about the current behavior has changed?

Issue Number:

=== RUN   TestAccFeed_DataSource_Feed_By_Name
=== PAUSE TestAccFeed_DataSource_Feed_By_Name
=== RUN   TestAccFeed_DataSource_Feed_By_Feed_Id
=== PAUSE TestAccFeed_DataSource_Feed_By_Feed_Id
=== RUN   TestAccFeed_basic
=== PAUSE TestAccFeed_basic
=== RUN   TestAccFeed_with_Project
=== PAUSE TestAccFeed_with_Project
=== RUN   TestAccFeed_softDeleteRecovery
--- PASS: TestAccFeed_softDeleteRecovery (38.72s)
=== CONT  TestAccFeed_DataSource_Feed_By_Name
=== CONT  TestAccFeed_basic
=== CONT  TestAccFeed_with_Project
=== CONT  TestAccFeed_DataSource_Feed_By_Feed_Id
--- PASS: TestAccFeed_basic (29.35s)
--- PASS: TestAccFeed_DataSource_Feed_By_Name (29.54s)
--- PASS: TestAccFeed_DataSource_Feed_By_Feed_Id (29.67s)
--- PASS: TestAccFeed_with_Project (63.02s)
PASS
ok      github.com/microsoft/terraform-provider-azuredevops/azuredevops/internal/acceptancetests        109.144s

=== RUN   TestAccFeedPermission_basic
=== PAUSE TestAccFeedPermission_basic
=== CONT  TestAccFeedPermission_basic
--- PASS: TestAccFeedPermission_basic (64.55s)
PASS
ok      github.com/microsoft/terraform-provider-azuredevops/azuredevops/internal/acceptancetests        71.262s

Does this introduce a change to go.mod, go.sum or vendor/?

• [ ] Yes
• [x] No

Does this introduce a breaking change?

• [ ] Yes
• [x] No

Any relevant logs, error output, etc?

(If it’s long, please paste to https://ghostbin.com/ and insert the link here.)

Other information