chore: Update test framework and packages

Author: phnx47

test/Unidecode.NET.Tests.csproj

@@ -1,13 +1,13 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>netcoreapp2.0</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.5.0" />
-    <PackageReference Include="xunit" Version="2.3.1" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="2.3.1" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.6.1" />
+    <PackageReference Include="xunit" Version="2.4.1" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.1" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\src\Unidecode.NET.csproj" />
   </ItemGroup>
-</Project>
+</Project>