Michał Kępień authored Apr 23, 2020 and Mark Andrews committed Jun 30, 2020

Make yaml.load_all() use yaml.SafeLoader to address a warning currently
emitted when bin/tests/system/dnstap/ydump.py is run:

    ydump.py:28: YAMLLoadWarning: calling yaml.load_all() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.
      for l in yaml.load_all(f.stdout):

(cherry picked from commit 62f631f7)