Include TestFramework.Extensions.dll in TestAdapter NuGet package for .NET Framework by @Youssef1313 in #6625 (backport to rel/3.10) (#6626)

Co-authored-by: Youssef1313 <[email protected]>

Author: youssef-backport-bot

eng/verify-nupkgs.ps1

@@ -21,7 +21,7 @@ function Confirm-NugetPackages {
     $expectedNumOfFiles = @{
         "MSTest.Sdk"                                  = 15
         "MSTest.TestFramework"                        = 162
-        "MSTest.TestAdapter"                          = 80
+        "MSTest.TestAdapter"                          = 81
         "MSTest"                                      = 14
         "MSTest.Analyzers"                            = 56
     }

src/Adapter/MSTest.TestAdapter/MSTest.TestAdapter.nuspec

@@ -133,6 +133,13 @@
     -->
     <file src="net462\Microsoft.VisualStudio.TestPlatform.TestFramework.dll" target="buildTransitive\net462\" />
 
+    <!-- 
+      Again, for .NET Framework, VSTest considers first MSTest.TestAdapter NuGet package, then considers VS installation.
+      This can result in TestFramework.Extensions.dll loaded from VS installation which mismatches the other assemblies.
+      See https://devdiv.visualstudio.com/DevDiv/_workitems/edit/2577652
+    -->
+    <file src="net462\Microsoft.VisualStudio.TestPlatform.TestFramework.Extensions.dll" target="buildTransitive\net462\" />
+
     <!-- Localization -->
     <!-- All TFMs share the same resx, copy only TestAdapter TestAdapter.PlatformServices, do NOT copy TestFramework and other dependency resource dlls. -->
     <file src="net462\**\Microsoft.VisualStudio.TestPlatform.*Adapter*.resources.dll" target="\buildTransitive\_localization\" />