Bump Microsoft.NET.Test.Sdk from 18.7.0 to 18.8.1 (#62)

---
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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GlennLibTests/GlennLibTests.csproj

@@ -10,7 +10,7 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.7.0" />
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.8.1" />
         <PackageReference Include="SixLabors.ImageSharp" Version="3.1.12" />
         <PackageReference Include="xunit" Version="2.7.0" />
         <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5">