tests/qtest/migration: Fix leak of migration tests data

When the migration-test is invoked with the '-p' flag (to run a single
test), the glib code won't call the destroy function for the
not-executed tests, causing the MigrationTest wrapper data to leak.

This doesn't affect make check, but affects debugging use-cases where
having a leak pop up in ASAN output is extra annoying.

Fix by adding the tests data to a list and freeing them all at the end
of migration-test execution. Any tests actually dispatched by glib
will have the destroy function called as usual.

Note that migration_test_add_suffix() is altered to call
migration_test_add() so that there's only one place adding the data to
the list.

Performance is not an issue at the moment, we have < 100 tests.

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Prasad Pandit <pjp@fedoraproject.org>
Link: https://lore.kernel.org/qemu-devel/20260311213418.16951-2-farosas@suse.de
Signed-off-by: Fabiano Rosas <farosas@suse.de>