[tests] Fix CRLF issues in System.CodeDom tests (#52540)

[tests] Fix CRLF issues in System.CodeDom tests (#52540)

Fixes: https://github.com/dotnet/runtime/issues/52107
When running `System.CodeDom` tests on windows and  targeting
`Browser`, the line ending differs between host and target systems.
Add helper `LineEndingsHelper.Normalize` method to update expected
strings.

The `Browser` Environment.NewLine is `\n`, while windows use `\r\n`.
https://github.com/dotnet/runtime/blob/5bd0edfe860e41bdfd690d3743e730594307292e/src/libraries/System.Private.CoreLib/src/System/Environment.UnixOrBrowser.cs#L51