Fixed PackageReferences on ObjectModel nupkg (#2660)

* Cherry-picked signing fixes from `master`

* Cherry-picked assembly loading fixes.

* Assembly loading is fixed in netcoreapp
* Added new signing certificates

* Fixed PackageReferences on ObjectModel nupkg

Author: Haplois

src/package/nuspec/TestPlatform.ObjectModel.nuspec

@@ -20,6 +20,8 @@
         <dependency id="NuGet.Frameworks" version="[5.0.0, )" />
       </group>
       <group targetFramework="netstandard2.0">
+        <dependency id="System.Runtime.InteropServices.RuntimeInformation" version="[4.0.0, )" />
+        <dependency id="System.Reflection.Metadata" version="[1.6.0, )" />
         <dependency id="NuGet.Frameworks" version="[5.0.0, )" />
       </group>
     </dependencies>