Syndication test (dotnet/corefx#24570)

Syndication test (dotnet/corefx#24570)

* Add SyndicationFeed Test case

* Add Xml compare Utils class

* Add CompareHelper for incomplete standard  feed files.

* Add issue dotnet/corefx#24571 and dotnet/corefx#24572 for failed test case.

* Change to use async method and mark failed case with active issue dotnet/corefx#24604

* Refactored code

* Add Feed files for AtomFees test

* Update

* Remove useless feeds files , and mark  the cause of the failed feeds files

* Update

* Modify test case from Async to Sync. Remove issue attribute dotnet/corefx#24571 dotnet/corefx#24572 dotnet/corefx#24604,
mark with new issue dotnet/corefx#24894.

Commit migrated from https://github.com/dotnet/corefx/commit/76baca36b0d678b9edb218c1aa08fec037a81a72