tests/qtest/migration-test: Only wait for serial output where migration succeeds

tests/qtest/migration-test: Only wait for serial output where migration succeeds

Waiting for the serial output can take a couple of seconds - and since
we're doing a lot of migration tests, this time easily sums up to
multiple minutes. But if a test is supposed to fail, it does not make
much sense to wait for the source to be in the right state first, so
we can skip the waiting here. This way we can speed up all tests where
the migration is supposed to fail. In the gitlab-CI gprov-gcov test,
each of the migration-tests now run two minutes faster!

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220819053802.296584-2-thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20220822165608.2980552-3-alex.bennee@linaro.org>