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.
Github-Pull: #12902
Rebased-From: 75d0e4c5444249f0cf81ed0494e2c2717170b695