- wrong fix especially now that test encoders work as expected
- touch up `HtmlHelperTest` and `RazorViewTest` since test encoders are now consistent
- remove references to old `Microsoft.Extensions.WebEncoders.Testing` package
This reverts commit a9d5876cd9.
- but leave `FormTagHelperTest` and `ValidationMessageTagHelperTest` cleanup alone
nit: use `string.Empty` in `HtmlHelperLinkGenerationTest`
