Disable xUnit2014 diagnostic

This diagnostic forces the use of Assert.ThrowsAsync for any async method,
however in our case we may want to test that a method will throw
synchronously to avoid regressing that behavior by moving to the async
portion of the method.

eng/CodeAnalysis.test.globalconfig

@@ -1681,7 +1681,7 @@ dotnet_diagnostic.xUnit2012.severity = warning
 dotnet_diagnostic.xUnit2013.severity = none
 
 # xUnit2014: Do not use throws check to check for asynchronously thrown exception
-dotnet_diagnostic.xUnit2014.severity = warning
+dotnet_diagnostic.xUnit2014.severity = none
 
 # xUnit2015: Do not use typeof expression to check the exception type
 dotnet_diagnostic.xUnit2015.severity = warning