This ensures the Carol and Dave nodes created in the single_hop series
of tests are synced to the latest chain tip generated by the miner
before creating the route that will eventually be used for payments.
This prevents a flake during CI tests where slow processing of blocks by
Carol could cause the generated route to have a final CLTV delta too
short for Dave to accept.
While at it, we switch the test to use the default CLTV delta provided
by QueryRoutes which is now the global default CLTV.
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