tests/functional: Rework the migration test to have target-specific files

We are going to move the tests for each target into separate subdirectories.
The migration test does not fit quite into this scheme, since it works
for multiple targets, but not all. Rework the test to have a common
test class, and target specific files with a target specific class, so
that this will fit better into the new scheme.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20250819112403.432587-2-thuth@redhat.com>
14 files changed