Don't use reflection on the private Exception._stackTraceString field

It was for tests only, but still…

See https://stackoverflow.com/questions/37093261/attach-stacktrace-to-exception-without-throwing-in-c-sharp-net/76777764#76777764

Author: 0xced

tests/Log4NetTextFormatterTest.Exception.verified.xml

@@ -1,5 +1,7 @@
 <log4net:event timestamp="2003-01-04T15:09:26.535+01:00" level="INFO" xmlns:log4net="http://logging.apache.org/log4net/schemas/log4net-events-1.2/">
   <log4net:message><![CDATA[Hello from Serilog]]></log4net:message>
   <log4net:exception><![CDATA[System.Exception: An error occurred
-  at Serilog.Formatting.Log4Net.Tests.Log4NetTextFormatterTest.BasicMessage_WithException() in Log4NetTextFormatterTest.cs:123]]></log4net:exception>
+  at Serilog.Formatting.Log4Net.Tests.Log4NetTextFormatterTest.BasicMessage_WithException() in Log4NetTextFormatterTest.cs:123
+--- End of stack trace from previous location ---
+]]></log4net:exception>
 </log4net:event>

tests/Log4NetTextFormatterTest.Log4JCompatibility.verified.xml

@@ -4,5 +4,7 @@
   </log4j:properties>
   <log4j:message><![CDATA[Hello from Serilog]]></log4j:message>
   <log4j:throwable><![CDATA[System.Exception: An error occurred
-  at Serilog.Formatting.Log4Net.Tests.Log4NetTextFormatterTest.BasicMessage_WithException() in Log4NetTextFormatterTest.cs:123]]></log4j:throwable>
+  at Serilog.Formatting.Log4Net.Tests.Log4NetTextFormatterTest.BasicMessage_WithException() in Log4NetTextFormatterTest.cs:123
+--- End of stack trace from previous location ---
+]]></log4j:throwable>
 </log4j:event>

tests/Log4NetTextFormatterTest.cs

@@ -2,7 +2,7 @@
 using System.Collections.Generic;
 using System.Globalization;
 using System.IO;
-using System.Reflection;
+using System.Runtime.ExceptionServices;
 using System.Threading.Tasks;
 using FluentAssertions;
 using Serilog.Core;
@@ -14,18 +14,6 @@
 
 namespace Serilog.Formatting.Log4Net.Tests;
 
-public static class ExceptionExtensions
-{
-    // See https://stackoverflow.com/questions/37093261/attach-stacktrace-to-exception-without-throwing-in-c-sharp-net/37605142#37605142
-    private static readonly FieldInfo StackTraceStringField = typeof(Exception).GetField("_stackTraceString", BindingFlags.NonPublic | BindingFlags.Instance) ?? throw new MissingFieldException(nameof(Exception), "_stackTraceString");
-
-    public static Exception SetStackTrace(this Exception exception, string stackTrace)
-    {
-        StackTraceStringField.SetValue(exception, stackTrace) ;
-        return exception;
-    }
-}
-
 public sealed class Log4NetTextFormatterTest : IDisposable
 {
     private readonly TextWriter _selfLogWriter;
@@ -311,7 +299,7 @@ public Task Log4JCompatibility(bool useStaticInstance)
         // Arrange
         using var output = new StringWriter();
         var logEvent = CreateLogEvent(
-            exception: new Exception("An error occurred").SetStackTrace("  at Serilog.Formatting.Log4Net.Tests.Log4NetTextFormatterTest.BasicMessage_WithException() in Log4NetTextFormatterTest.cs:123"),
+            exception: ExceptionDispatchInfo.SetRemoteStackTrace(new Exception("An error occurred"), "  at Serilog.Formatting.Log4Net.Tests.Log4NetTextFormatterTest.BasicMessage_WithException() in Log4NetTextFormatterTest.cs:123"),
             properties: new LogEventProperty("π", new ScalarValue(3.14m))
         );
         var formatter = useStaticInstance ? Log4NetTextFormatter.Log4JFormatter : new Log4NetTextFormatter(c => c.UseLog4JCompatibility());
@@ -440,7 +428,7 @@ public Task Exception()
     {
         // Arrange
         using var output = new StringWriter();
-        var logEvent = CreateLogEvent(exception: new Exception("An error occurred").SetStackTrace("  at Serilog.Formatting.Log4Net.Tests.Log4NetTextFormatterTest.BasicMessage_WithException() in Log4NetTextFormatterTest.cs:123"));
+        var logEvent = CreateLogEvent(exception: ExceptionDispatchInfo.SetRemoteStackTrace(new Exception("An error occurred"), "  at Serilog.Formatting.Log4Net.Tests.Log4NetTextFormatterTest.BasicMessage_WithException() in Log4NetTextFormatterTest.cs:123"));
         var formatter = new Log4NetTextFormatter();
 
         // Act