Bump System.IO.Abstractions.TestingHelpers from 20.0.4 to 20.0.28

Bumps [System.IO.Abstractions.TestingHelpers](https://github.com/TestableIO/System.IO.Abstractions) from 20.0.4 to 20.0.28.
- [Release notes](https://github.com/TestableIO/System.IO.Abstractions/releases)
- [Commits](TestableIO/System.IO.Abstractions@v20.0.4...v20.0.28)

---
updated-dependencies:
- dependency-name: System.IO.Abstractions.TestingHelpers
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

tests/LocalAppVeyor.Engine.UnitTests/LocalAppVeyor.Engine.UnitTests.csproj

@@ -10,7 +10,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
-    <PackageReference Include="System.IO.Abstractions.TestingHelpers" Version="20.0.4" />
+    <PackageReference Include="System.IO.Abstractions.TestingHelpers" Version="20.0.28" />
     <PackageReference Include="xunit" Version="2.6.4" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.5.6">
       <PrivateAssets>all</PrivateAssets>