Bump Microsoft.NET.Test.Sdk from 15.0.0 to 15.9.0 (#413)

Bumps [Microsoft.NET.Test.Sdk](https://github.com/microsoft/vstest) from 15.0.0 to 15.9.0.
- [Release notes](https://github.com/microsoft/vstest/releases)
- [Commits](microsoft/vstest@v15.0.0...v15.9.0)

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

Author: dependabot[bot]

System.IO.Abstractions.TestingHelpers.Tests/System.IO.Abstractions.TestingHelpers.Tests.csproj

@@ -41,7 +41,7 @@
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFramework)' != 'net40'">
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.0.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.9.0" />
     <PackageReference Include="Moq" Version="4.10.0" />
   </ItemGroup>