Test project: Enable netcore3.1 testing, switch win64 Snap7 and use custom port.

- Add netcoreapp3.1 target framework, allowing this to run on linux and macos as well.
- Switch windows snap7 DLL to 64bit version 1.4.2. This also improves UnitTest stability (reduces false positives) on the CI (including appveyor)
- Changing the port used for S7NetTests when communicating with Snap7 to a value > 1000 allows tests to run on Linux without elevated privileges.

S7.Net.UnitTest/S7.Net.UnitTest.csproj

@@ -1,15 +1,17 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net452</TargetFrameworks>
+    <TargetFrameworks>net452;netcoreapp3.1</TargetFrameworks>
 
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>Properties\S7.Net.snk</AssemblyOriginatorKeyFile>
     <IsPackable>false</IsPackable>
     <Copyright>Copyright ©  2014</Copyright>
+    <PlatformTarget>x64</PlatformTarget>
   </PropertyGroup>
-
+  
   <ItemGroup>
+    <PackageReference Include="GitHubActionsTestLogger" Version="1.1.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.7.0" />
     <PackageReference Include="MSTest.TestAdapter" Version="2.1.2" />
     <PackageReference Include="MSTest.TestFramework" Version="2.1.2" />
@@ -23,14 +25,13 @@
     <ProjectReference Include="..\S7.Net\S7.Net.csproj" />
   </ItemGroup>
 
-  <ItemGroup>
+  <ItemGroup Condition="'$(TargetFramework)'=='net452'">
     <Reference Include="System.ServiceProcess" />
   </ItemGroup>
 
   <ItemGroup>
-    <None Update="snap7.dll">
+    <None Update="runtimes\win-x64\native\snap7.dll" Link="snap7.dll">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
   </ItemGroup>
-
 </Project>