Bump Microsoft.NET.Test.Sdk from 18.3.0 to 18.6.0 (#26)

---
updated-dependencies:
- dependency-name: Microsoft.NET.Test.Sdk
  dependency-version: 18.6.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]

Directory.Packages.props

@@ -5,7 +5,7 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageVersion Include="Microsoft.CodeAnalysis.NetAnalyzers" Version="8.0.0" />
-    <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.3.0" />
+    <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.6.0" />
     <PackageVersion Include="Microsoft.SourceLink.GitHub" Version="8.0.0" />
     <PackageVersion Include="StyleCop.Analyzers" Version="1.2.0-beta.556" />
     <PackageVersion Include="xunit.runner.visualstudio" Version="3.1.5" />