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