test: fix warning about yaml.load()
test: fix warning about yaml.load()

Test flush-all.test.py uses PyYAML and loads YAML file with yaml.load().
However, using yaml.load() is unsafe, and library shows warning in test
log:

```
<string>:19: YAMLLoadWarning: calling yaml.load() without Loader=... is
deprecated, as the default Loader is unsafe. Please read
https://msg.pyyaml.org/load
```

For more details see PyYAML wiki [1].

1. https://github.com/yaml/pyyaml/wiki/PyYAML-yaml.load(input)-Deprecation

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