Update NUnit to 3.16.1

Source/Platforms/DotNet/NUnit/agents/net40/nunit.agent.addins

@@ -1 +0,0 @@
-../../ # refer to the directory containing the engine and runner

Source/Platforms/DotNet/NUnit/agents/net462/nunit-agent.exe.config

@@ -0,0 +1,41 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <!--
+   Nunit-agent only runs under .NET 2.0 or higher. 
+   The setting    useLegacyV2RuntimeActivationPolicy only applies 
+   under .NET 4.0 and permits use of mixed mode assemblies, 
+   which would otherwise not load correctly. 
+  -->
+  <startup useLegacyV2RuntimeActivationPolicy="true">
+    <!--
+     Nunit-agent is normally run by the console or gui
+     runners and not independently. In normal usage, 
+     the runner specifies which runtime should be used.
+     
+     Do NOT add any supportedRuntime elements here, 
+     since they may prevent the runner from controlling 
+     the runtime that is used!
+    -->
+  </startup>
+
+  <runtime>
+    <!-- Ensure that test exceptions don't crash NUnit -->
+    <legacyUnhandledExceptionPolicy enabled="1" />
+
+    <!--
+     Since legacyUnhandledExceptionPolicy keeps the console from being killed even though an NUnit framework
+     test worker thread is killed, this is needed to prevent a hang. NUnit framework can only handle these
+     exceptions when this config element is present. (Or if future versions of NUnit framework drop support
+     for partial trust which would enable it to use [HandleProcessCorruptedStateExceptions].)
+    -->
+    <legacyCorruptedStateExceptionsPolicy enabled="true" />
+
+    <!-- Run partial trust V2 assemblies in full trust under .NET 4.0 -->
+    <loadFromRemoteSources enabled="true" />
+
+    <!-- Enable reading source information from Portable and Embedded PDBs when running applications -->
+    <!-- built against previous .NET Framework versions on .NET Framework 4.7.2 -->
+    <AppContextSwitchOverrides value="Switch.System.Diagnostics.IgnorePortablePDBsInStackTraces=false" />
+  </runtime>
+  
+</configuration>