Update xunit to 2.4.0

Author: bording

LibGit2Sharp.Tests/LibGit2Sharp.Tests.csproj

@@ -14,9 +14,9 @@
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.8.0" />
     <PackageReference Include="Moq" Version="4.8.2" />
-    <PackageReference Include="xunit" Version="2.3.1" />
-    <PackageReference Include="xunit.runner.console" Version="2.3.1" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="2.3.1" />
+    <PackageReference Include="xunit" Version="2.4.0" />
+    <PackageReference Include="xunit.runner.console" Version="2.4.0" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.0" />
     <PackageReference Include="xunit.skippablefact" Version="1.3.3" />
   </ItemGroup>