refactor: remove static uses of PrettyMarkupFormatter (#1609)

PrettyMarkupFormatter is not thread safe

VerifyTests/Verify.Bunit#60

Author: SimonCropp

src/bunit.web/Diffing/DiffMarkupFormatter.cs

@@ -16,6 +16,7 @@ public class DiffMarkupFormatter : PrettyMarkupFormatter, IMarkupFormatter
 	/// The <see cref="DiffMarkupFormatter"/> is not thread safe, so using this singleton
 	/// instance to format elements may not result in the desired effect.
 	/// </remarks>
+	[Obsolete("This instance is not thread safe, use a new instance instead.")]
 	public static new readonly DiffMarkupFormatter Instance = new();
 
 	/// <summary>

src/bunit.web/Extensions/NodePrintExtensions.cs

@@ -109,6 +109,7 @@ public static string ToMarkupElementOnly(this IElement element)
 		if (element is null)
 			throw new ArgumentNullException(nameof(element));
 
+		var diffMarkupFormatter = new DiffMarkupFormatter();
 		var result = new StringBuilder();
 		result.Append(Symbols.LessThan);
 
@@ -120,7 +121,7 @@ public static string ToMarkupElementOnly(this IElement element)
 
 		foreach (var attribute in element.Attributes)
 		{
-			result.Append(' ').Append(DiffMarkupFormatter.Instance.ConvertToString(attribute));
+			result.Append(' ').Append(diffMarkupFormatter.ConvertToString(attribute));
 		}
 
 		if (element.HasChildNodes)