Add testing specific provides to allow simple mapping from source to installable packages

We install kernels as expressed by linux by installation of packages from linux-meta based on the flavour names. It is not entirely trivial to map the source package name to a flavour, it becomes doubly hard when we have variants applied and the primary variant is not in this package. For example when linux-oracle is expressing -lts-18.04 only and linux-oracle-5.0 is expressing -- we end up installing linux-image-oracle for both packages and only test the latter one.

Add new Provides to the appropriate meta packages so we can always directly select an appropriate meta package without knowing its name:

  Package: linux-oracle-lts-18.04
  Provides: kernel-testing--linux-oracle--full--oracle

We should also include a preferred option for ADT as it also is unaware that the kernel package could actually emit more than one kernel flavour.

  Package: linux-generic-hwe-18.04-edge
  Provides: kernel-testing--linux-hwe-5.3--full--generic, kernel-testing--linux-hwe-5.3--full--preferred