Bump Microsoft.NET.Test.Sdk from 18.7.0 to 18.8.1 (#68)

---
updated-dependencies:
- dependency-name: Microsoft.NET.Test.Sdk
  dependency-version: 18.8.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]

Directory.Packages.props

@@ -33,7 +33,7 @@
   </ItemGroup>
   <ItemGroup>
     <!-- Test + sample -->
-    <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.7.0" />
+    <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.8.1" />
     <PackageVersion Include="coverlet.collector" Version="10.0.1" />
     <PackageVersion Include="xunit" Version="2.9.3" />
     <PackageVersion Include="xunit.runner.visualstudio" Version="3.1.5" />