test: speed up integration test harness initialization
test: speed up integration test harness initialization

This commit speeds up the integration test initialization by launching
the processes of the two seeder nodes concurrently rather than
serially.

Additionally, the harness will now block until the wallets of both the
seeder nodes are fully synced up.

LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd,bryanvu/lnd,AndrewSamokhvalov/lnd,ddustin/lnd,bryanvu/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd,ddustin/lnd