Update NuGet packages.

Author: bgrainger

tests/Directory.Build.props

@@ -1,7 +1,7 @@
 <Project>
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
-    <PackageReference Include="xunit" Version="2.6.6" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="2.5.6" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.9.0" />
+    <PackageReference Include="xunit" Version="2.7.0" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.5.7" />
   </ItemGroup>
 </Project>