Fix failing xNodeBuilder tests (dotnet/corefx#35140)

Fix failing xNodeBuilder tests (dotnet/corefx#35140)

1 of the tests is failing because float.MaxValue.ToString() changed and it had hardcoded the previous value.

4 of the tests failed because it was doing a string comparison on the type name of Encoding.UTF8, which a while ago changed to be a sealed derived type and thus is no longer exactly "System.Text.UTF8Encoding".  My only explanation for why these just started failing is that we weren't previously running them?  Might have something to do with the lack of a Configurations.props file, which I had to add in order to run the tests locally.

Commit migrated from https://github.com/dotnet/corefx/commit/a27099da194460992a9b8c098f1b5f07837577e0