Reenable test on s390x in tests.proj (#99453)

We're using a new enough nuget now.

Author: akoeplinger

src/libraries/tests.proj

@@ -138,8 +138,6 @@
     <!-- LibraryImportGenerator runtime tests build depends pulling down a pre-built nethost binary, which is not available for s390x. -->
     <ProjectExclusions Include="$(MSBuildThisFileDirectory)System.Runtime.InteropServices\tests\LibraryImportGenerator.Tests\LibraryImportGenerator.Tests.csproj" />
     <ProjectExclusions Include="$(MSBuildThisFileDirectory)System.Runtime.InteropServices\tests\ComInterfaceGenerator.Tests\ComInterfaceGenerator.Tests.csproj" />
-    <!-- LibraryImportGenerator unit tests fail since NuGet 5.6.0 signature verification does not work on big-endian systems (needs >= 5.11.0). -->
-    <ProjectExclusions Include="$(MSBuildThisFileDirectory)System.Runtime.InteropServices\tests\LibraryImportGenerator.UnitTests\LibraryImportGenerator.Unit.Tests.csproj" />
     <ProjectExclusions Include="$(MSBuildThisFileDirectory)System.Runtime.InteropServices\tests\ComInterfaceGenerator.Unit.Tests\ComInterfaceGenerator.Unit.Tests.csproj" />
     <ProjectExclusions Include="$(MSBuildThisFileDirectory)System.Runtime.InteropServices.JavaScript\tests\JSImportGenerator.UnitTest\JSImportGenerator.Unit.Tests.csproj" />
   </ItemGroup>