Add System.Runtime.CompilerServices.Unsafe to BenchmarkDotNet.Tests

Needed for Verify.Xunit in order to correctly resolve dependencies on net46x/Windows

Author: AndreyAkinshin

tests/BenchmarkDotNet.Tests/BenchmarkDotNet.Tests.csproj

@@ -28,6 +28,7 @@
     <ProjectReference Include="..\..\src\BenchmarkDotNet.Diagnostics.Windows\BenchmarkDotNet.Diagnostics.Windows.csproj" />
     <PackageReference Include="Microsoft.NETCore.Platforms" Version="6.0.0" />
     <PackageReference Include="System.Memory" Version="4.5.5" />
+    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="6.0.0" />
     <Reference Include="System.Runtime" />
     <Reference Include="System.Threading.Tasks" />
     <Reference Include="System" />