fix: JUnit reporter was encoding xml files in UTF-16 instead of UTF-8 (#4001)

* fix: JUnit reporter

* Enforce UTF-8 instead of UTF-16

Author: rchoffardet

TUnit.Engine/Xml/JUnitXmlWriter.cs

@@ -33,7 +33,7 @@ public static string GenerateXml(
                 ?.FrameworkDisplayName
             ?? RuntimeInformation.FrameworkDescription;
 
-        using var stringWriter = new StringWriter();
+        using var stringWriter = new Utf8StringWriter();
         var settings = new XmlWriterSettings
         {
             Indent = true,
@@ -341,6 +341,11 @@ private static TestSummary CalculateSummary(IEnumerable<TestNodeUpdateMessage> t
     }
 }
 
+file sealed class Utf8StringWriter : StringWriter
+{
+    public override Encoding Encoding => Encoding.UTF8;
+}
+
 internal sealed class TestSummary
 {
     public int Total { get; set; }