Bump Microsoft.NET.Test.Sdk from 15.6.0 to 15.8.0

Bumps [Microsoft.NET.Test.Sdk](https://github.com/microsoft/vstest) from 15.6.0 to 15.8.0.
- [Release notes](https://github.com/microsoft/vstest/releases)
- [Commits](https://github.com/microsoft/vstest/compare/v15.6.0...v15.8.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

OhmGraphite.Test/OhmGraphite.Test.csproj

@@ -5,7 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.6.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.8.0" />
     <PackageReference Include="xunit" Version="2.3.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.3.1" />
     <DotNetCliToolReference Include="dotnet-xunit" Version="2.3.1" />