pyln: Set the bitcoin datadir when running tests

Telling `lightningd` to pass a `-datadir` to `bitcoin-cli` so it doesn't go
snooping where it doesn't belong (i.e., the user's home directory and config).

Changelog-None
Suggested-by: Simon Vrouwe <@SimonVrouwe>
Signed-off-by: Christian Decker <@cdecker>

contrib/pyln-testing/pyln/testing/utils.py

@@ -489,6 +489,9 @@ class LightningD(TailableProc):
             'ignore-fee-limits': 'false',
             'bitcoin-rpcuser': BITCOIND_CONFIG['rpcuser'],
             'bitcoin-rpcpassword': BITCOIND_CONFIG['rpcpassword'],
+
+            # Make sure we don't touch any existing config files in the user's $HOME
+            'bitcoin-datadir': lightning_dir,
         }
 
         for k, v in opts.items():

tests/plugins/bitcoin/part1.py

@@ -33,5 +33,6 @@ def getchaininfo(plugin, **kwargs):
 plugin.add_option("bitcoin-rpcuser", "", "")
 plugin.add_option("bitcoin-rpcpassword", "", "")
 plugin.add_option("bitcoin-rpcport", "", "")
+plugin.add_option("bitcoin-datadir", "", "")
 
 plugin.run()