Bump Microsoft.NET.Test.Sdk from 18.7.0 to 18.8.1

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

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

props/SharedTestProps.props

@@ -7,7 +7,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.7.0"/>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.8.1"/>
     <PackageReference Include="Shouldly" Version="4.3.0"/>
     <PackageReference Include="xunit" Version="2.9.3"/>
     <PackageReference Include="xunit.runner.visualstudio" Version="3.1.3"/>