Update MS test sdk and Xunit test runner

Author: JimBobSquarePants

msbuild/props/SixLabors.Tests.props

@@ -20,12 +20,11 @@
                       Condition="'$(IsCodeCoverage)'=='true'"/>
 
     <PackageReference Include="Microsoft.NET.Test.Sdk"
-                      Version="16.8.0"
-                      IsImplicitlyDefined="true"
-                      Condition="'$(TargetFramework)' != 'netcoreapp2.0'"/>
+                      Version="17.4.1"
+                      IsImplicitlyDefined="true"/>
 
     <PackageReference Include="xunit.runner.visualstudio"
-                      Version="2.4.3"
+                      Version="2.4.5"
                       IsImplicitlyDefined="true" />
 
     <Using Include="Xunit" Condition="'$(ImplicitUsings)'=='enable' OR '$(ImplicitUsings)'=='true'"/>