tests/block-job-txn: Don't start block job before adding to txn

tests/block-job-txn: Don't start block job before adding to txn

Previously, before test_block_job_start returns, the job can already
complete, as a result, the transactional state of other jobs added to
the same txn later cannot be handled correctly.

Move the block_job_start() calls to callers after
block_job_txn_add_job() calls.

Signed-off-by: Fam Zheng <famz@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>