corrected the manifest for gadget testing (Bugfix) (#1696)

corrected the manifest for gadget testing

contrib/checkbox-ce-oem/checkbox-provider-ce-oem/units/gadget/jobs.pxu

@@ -48,7 +48,7 @@ imports:
   from com.canonical.certification import lsb
   from com.canonical.plainbox import manifest
 requires:
-  manifest.has_interface_been_defined == 'True'
+  manifest.need_gadget_interface_test == 'True'
   lsb.distributor_id == 'Ubuntu Core'
 category_id: com.canonical.certification::gadget
 estimated_duration: 5s
@@ -68,7 +68,7 @@ imports:
   from com.canonical.certification import lsb
   from com.canonical.plainbox import manifest
 requires:
-  manifest.has_interface_been_defined == 'True'
+  manifest.need_gadget_interface_test == 'True'
   lsb.distributor_id == 'Ubuntu Core'
 category_id: com.canonical.certification::gadget
 estimated_duration: 5s