Note this is apart from the Code Signing certificate you create from the Project Property Pages. Also, in creating an APPX package this (Test) Certificate must be intact or else the package creation will fail.
Just had another example with this: Occurred when was developing app on one system and saved to TFS. Then did Get on another machine. When I build package got: 1>C:\Program Files (x86)\MSBuild\Microsoft\VisualStudio\v14.0\AppxPackage\Microsoft.AppXPackage.Targets(2519,5): error APPX0104: Certificate file 'myapp_TemporaryKey.pfx' not found. 1>C:\Program Files (x86)\MSBuild\Microsoft\VisualStudio\v14.0\AppxPackage\Microsoft.AppXPackage.Targets(2519,5): error APPX0102: A certificate with thumbprint '3AB810C6505989B871534A66A6286E19ABFB8DD6' that is specified in the project cannot be found in the certificate store. Please specify a valid thumbprint in the project file. 1>C:\Program Files (x86)\MSBuild\Microsoft\VisualStudio\v14.0\AppxPackage\Microsoft.AppXPackage.Targets(2519,5): error APPX0107: The certificate specified is not valid for signing. For more information about valid certificates, see http://go.microsoft.com/fwlink/?LinkID=241478.
I repeat: Note this is apart from the Code Signing certificate you create from the Project Property Pages. THE CODE SIGNING .PFX FILE THAT YOU CAN CONFIGURE IN THE PROJECT PROPERTY DEBUG PAGE IS A SEPARATE FILE TO THE APPMANIFEST ONE. PACKAGE BUILD WILL FAIL IF YOU MAKE THE APPMANIFEST ONE THE CODE SIGNING ONE. At least that is what I found!