Cleanup test project

* Update .NET Core target from .NET Core 1.0 to .NET Core 3.1 (LTS)
* Use latest versions of Microsoft.NET.Test.Sdk, xunit.runner.visualstudio and xunit
* Remove everything unneeded to build and run tests
* Update Visual Studio image from 2017 to 2019 on AppVeyor

Running tests now work in both IDEs and with `dotnet test` command line invocation.

appveyor.yml

@@ -1,6 +1,6 @@
 version: '{build}'
 skip_tags: true
-image: Visual Studio 2017
+image: Visual Studio 2019
 configuration: Release
 install:
   - ps: mkdir -Force ".\build\" | Out-Null

test/Serilog.Enrichers.Environment.Tests/Serilog.Enrichers.Environment.Tests.csproj

@@ -1,38 +1,18 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>netcoreapp1.0;net46</TargetFrameworks>
-    <AssemblyName>Serilog.Enrichers.Environment.Tests</AssemblyName>
-    <AssemblyOriginatorKeyFile>../../assets/Serilog.snk</AssemblyOriginatorKeyFile>
-    <SignAssembly>true</SignAssembly>
-    <PublicSign Condition=" '$(OS)' != 'Windows_NT' ">true</PublicSign>
-    <PackageId>Serilog.Enrichers.Environment.Tests</PackageId>
-    <GenerateRuntimeConfigurationFiles>true</GenerateRuntimeConfigurationFiles>
-    <PackageTargetFallback Condition=" '$(TargetFramework)' == 'netcoreapp1.0' ">$(PackageTargetFallback);dnxcore50;portable-net45+win8</PackageTargetFallback>
-    <RuntimeFrameworkVersion Condition=" '$(TargetFramework)' == 'netcoreapp1.0' ">1.0.4</RuntimeFrameworkVersion>
+    <TargetFrameworks>netcoreapp3.1;net46</TargetFrameworks>
+    <IsPackable>false</IsPackable>
   </PropertyGroup>
 
-  <ItemGroup>
-    <None Include="App.config" />
-  </ItemGroup>
-
   <ItemGroup>
     <ProjectReference Include="..\..\src\Serilog.Enrichers.Environment\Serilog.Enrichers.Environment.csproj" />
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.0.0-preview-20170106-08" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="2.2.0-beta5-build1225" />
-    <PackageReference Include="xunit" Version="2.2.0-beta5-build3474" />
-  </ItemGroup>
-
-  <ItemGroup Condition=" '$(TargetFramework)' == 'net46' ">
-    <Reference Include="System" />
-    <Reference Include="Microsoft.CSharp" />
-  </ItemGroup>
-
-  <ItemGroup>
-    <Service Include="{82a7f48d-3b50-4b1e-b82e-3ada8210c358}" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.8.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3" PrivateAssets="all" />
+    <PackageReference Include="xunit" Version="2.4.1" />
   </ItemGroup>
 
 </Project>