Update Microsoft.NET.Test.Sdk to 15.8.0

Author: bording

LibGit2Sharp.Tests/LibGit2Sharp.Tests.csproj

@@ -12,7 +12,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.6.0" />
+    <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" />