Bump xunit from 2.4.0 to 2.4.2 in /tests/NLog.UnitTests

Bumps [xunit](https://github.com/xunit/xunit) from 2.4.0 to 2.4.2.
- [Release notes](https://github.com/xunit/xunit/releases)
- [Commits](xunit/xunit@2.4...2.4.2)

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

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

Author: dependabot[bot]

tests/NLog.UnitTests/NLog.UnitTests.csproj

@@ -37,7 +37,7 @@
   <ItemGroup>
     <PackageReference Include="Newtonsoft.Json" Version="13.0.1" /> <!-- Because of OWIN + NetCoreApp3.1 -->
     <PackageReference Include="NSubstitute" Version="4.4.0" />
-    <PackageReference Include="xunit" Version="2.4.0" />
+    <PackageReference Include="xunit" Version="2.4.2" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.4.0" />
     <PackageReference Include="Microsoft.TestPlatform.ObjectModel" Version="17.4.0" Condition="'$(OS)' != 'Windows_NT'" />