deps(test): update dependency microsoft.net.test.sdk to 18.8.1

Author: renovate[bot]

test/Directory.Build.props

@@ -10,7 +10,7 @@
 
     <ItemGroup>
         <PackageReference Include="FluentAssertions" Version="8.10.0" />
-        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.7.0" />
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.8.1" />
         <PackageReference Include="xunit.v3" Version="3.2.2" />
         <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5">
             <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>