Prepare for arcade changes that enable testing of meta-packages (#55976)

Author: ericstj

src/libraries/pkg/test/packageSettings/Microsoft.Windows.Compatibility/workaroundDowngrade.targets

@@ -0,0 +1,6 @@
+<Project>
+  <ItemGroup>
+    <!-- We cannot reference the current build of runtime.native.System.IO.Ports because its built across multiple machines -->
+    <PackageReference Condition="'$(TargetFrameworkIdentifier)' != '.NETFramework'" Include="runtime.native.System.IO.Ports" Version="$(runtimenativeSystemIOPortsVersion)" NoWarn="NU1605" />
+  </ItemGroup>
+</Project>

src/libraries/pkg/test/testPackages.proj

@@ -33,6 +33,8 @@
     </AllPackagesFilePath>
     <AllPackages Include="@(AllPackagesFilePath->'%(PackageName)')" />
 
+    <!-- runtime.native.System.IO.Ports requires all runtime legs to be built in order to test -->
+    <ExcludePackages Include="runtime.native.System.IO.Ports" />
     <ExcludePackages Condition="'$(PackagesToTest)' != ''" Include="@(AllPackages)" Exclude="@(PackagesToTest)" />
     <TestPackages Include="@(AllPackages)" Exclude="@(ExcludePackages)" />
     <TestPackagesPath Include="@(TestPackages->'%(PackagePath)')" />