Bump xunit from 2.6.6 to 2.7.0 (#253)

Author: dependabot[bot]

Directory.Packages.props

@@ -8,7 +8,7 @@
   <ItemGroup>
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.9.0" />
     <PackageVersion Include="xunit.runner.visualstudio" Version="2.5.7" />
-    <PackageVersion Include="xunit" Version="2.6.6" />
+    <PackageVersion Include="xunit" Version="2.7.0" />
   </ItemGroup>
   <ItemGroup>
     <GlobalPackageReference Include="CSharpIsNullAnalyzer" Version="0.1.495" />