pytest: move regtest argument into config file.

It's not optional for our test setup, and this makes it easier to invoke
bitcoin-cli manually, for example.

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

tests/utils.py

@@ -12,6 +12,7 @@ from ephemeral_port_reserve import reserve
 
 
 BITCOIND_CONFIG = {
+    "regtest": 1,
     "rpcuser": "rpcuser",
     "rpcpassword": "rpcpass",
     "rpcport": 18332,
@@ -236,7 +237,6 @@ class BitcoinD(TailableProc):
             '-datadir={}'.format(bitcoin_dir),
             '-printtoconsole',
             '-server',
-            '-regtest',
             '-logtimestamps',
             '-nolisten',
         ]