[qa] Delete cookie file before starting node

If a cookie file exists in a datadir prior to node startup, it must have
been leftover from a prior unclean shutdown. As bitcoind will overwrite
it anyway, delete it before starting up to prevent the test framework
from inadvertently trying to connect using stale credentials.

test/functional/test_framework/test_node.py

@@ -20,6 +20,7 @@ from .authproxy import JSONRPCException
 from .util import (
     append_config,
     assert_equal,
+    delete_cookie_file,
     get_rpc_proxy,
     rpc_url,
     wait_until,
@@ -105,6 +106,10 @@ class TestNode():
             extra_args = self.extra_args
         if stderr is None:
             stderr = self.stderr
+        # Delete any existing cookie file -- if such a file exists (eg due to
+        # unclean shutdown), it will get overwritten anyway by bitcoind, and
+        # potentially interfere with our attempt to authenticate
+        delete_cookie_file(self.datadir)
         self.process = subprocess.Popen(self.args + extra_args, stderr=stderr, *args, **kwargs)
         self.running = True
         self.log.debug("bitcoind started, waiting for RPC to come up")