Try getting .NET 4.6.2 test targets to run

Author: nblumhardt

src/Serilog.Extensions.Logging/Serilog.Extensions.Logging.csproj

@@ -29,6 +29,10 @@
     <PackageReference Include="Nullable" Version="1.3.1" PrivateAssets="All" />
   </ItemGroup>
 
+  <ItemGroup Condition=" '$(TargetFramework)' == 'net462' ">
+    <PackageReference Include="System.ValueTuple" Version="4.6.1" />
+  </ItemGroup>
+
   <PropertyGroup Condition=" '$(TargetFramework)' == 'netstandard2.1' ">
     <DefineConstants>$(DefineConstants);FEATURE_ITUPLE</DefineConstants>
   </PropertyGroup>