tests: change --override-fee-rate to --override-fee-rates and use in tests.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/utils.py

@@ -238,6 +238,7 @@ class LightningD(TailableProc):
             '--bitcoin-datadir={}'.format(bitcoin_dir),
             '--lightning-dir={}'.format(lightning_dir),
             '--port={}'.format(port),
+            '--override-fee-rates=15000/7500/1000',
             '--network=regtest'
         ]
         if DEVELOPER: