This clears up the following deprecation message when running the test:

test/functional/feature_dbcrash.py:270: DeprecationWarning: The 'warn'
method is deprecated, use 'warning' instead
  self.log.warn("Node %d never crashed during utxo flush!", i)

Git grepping indicates that this was the last remaining use of
`log.warn` in the functional tests.

./test/functional/test_runner.py feature_dbcrash