tests: prepare for bitcoind 0.16.

I noticed some breakage with git master:
1. getinfo no longer supported (for us, use getblockchaininfo)
2. generate no longer supported (use generatetoaddress)

Both these options are supported at least in 0.15, too.

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

tests/benchmark.py

@@ -29,10 +29,10 @@ def bitcoind():
     # Make sure we have segwit and some funds
     if info['blocks'] < 432:
         logging.debug("SegWit not active, generating some more blocks")
-        bitcoind.rpc.generate(432 - info['blocks'])
+        bitcoind.generate_block(432 - info['blocks'])
     elif info['balance'] < 1:
         logging.debug("Insufficient balance, generating 1 block")
-        bitcoind.rpc.generate(1)
+        bitcoind.generate_block(1)
 
     yield bitcoind