tests/qtest/migration: Stop calling everything "test"

tests/qtest/migration: Stop calling everything "test"

Test frameworks usually prefix "test_" to the entry point of the test
code. Having every function prefixed with test_ makes it hard to
understand the code and to grep for the actual tests.

Remove the "test" prefix from everything that is not a test.

In order to still keep some namespacing, stick to the "migrate_"
prefix, which is the most used currently.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>