Bump xunit from 2.8.1 to 2.9.0 in /tests/NLog.Web.Tests (#1049)

Bumps xunit from 2.8.1 to 2.9.0.

---
updated-dependencies:
- dependency-name: xunit
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

tests/NLog.Web.Tests/NLog.Web.Tests.csproj

@@ -15,7 +15,7 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
     <PackageReference Include="NSubstitute" Version="4.4.0" />
     <PackageReference Include="System.ValueTuple" Version="4.5.0" />
-    <PackageReference Include="xunit" Version="2.8.1" />
+    <PackageReference Include="xunit" Version="2.9.0" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.8.0">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>