Disable AppxPackageSigning for the UnitTest project

[jstedfast]

Do we need this??

The current "Temporary" certificate has expired and is causing the builds to fail:

Also grouped the Certificate/Signing properties together to make it a bit easier to see all the related configuration properties.

  C:\Program Files\Microsoft Visual Studio\2022\Enterprise\MSBuild\Microsoft\VisualStudio\v17.0\AppxPackage\Microsoft.AppXPackage.Targets(3574,5): error APPX0108: The certificate specified has expired. For more information about renewing certificates, see http://go.microsoft.com/fwlink/?LinkID=241478. [D:\a\XamlBehaviors\XamlBehaviors\src\BehaviorsSDKManaged\ManagedUnitTests\ManagedUnitTests.csproj]

    36 Warning(s)
    1 Error(s)

Time Elapsed 00:04:30.76
Error: Process completed with exit code 1.