Bump Microsoft.NET.Test.Sdk from 17.9.0 to 18.4.0 (#57)

---
updated-dependencies:
- dependency-name: Microsoft.NET.Test.Sdk
  dependency-version: 18.4.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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GlennLibTests/GlennLibTests.csproj

@@ -10,7 +10,7 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.9.0" />
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.4.0" />
         <PackageReference Include="SixLabors.ImageSharp" Version="3.1.7" />
         <PackageReference Include="xunit" Version="2.7.0" />
         <PackageReference Include="xunit.runner.visualstudio" Version="2.5.7">