debian/tests: Run amd64 tests on either amd64 or generic flavour

* Change the linux-image-amd64 dependency to -amd64 | -generic:amd64

* Replace the LOCALVERSION variable with a SUPPORTED_FLAVOURS
  variable, and have test-common loop over the flavours

* Change the dpkg-query pipeline that finds the kernel version
  string to avoid assumptions about ordering and versioning of
  dependencies

This should allow the tests to run on Ubuntu or its derivatives
without changes.

Signed-off-by: Ben Hutchings <benh@debian.org>