samples: create subscription with filtering enabled

[anguillanneuf]

Towards b/214396829.

The test output string looks something like 'Created subscription with filtering enabled: name: "projects/tz-playground-bigdata/subscriptions/subscription-test-su...: 604800\n}\nexpiration_policy {\n ttl {\n seconds: 2678400\n }\n}\nfilter: "attributes.author=\\"unknown\\""\n\n', which is the reason for assert '"attributes.author=\\"unknown\\""' in out in the test.

[snippet-bot]

Here is the summary of changes.

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• samples/snippets/subscriber.py:223, tag pubsub_create_subscription_with_filter

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[dandhlee]

LGTM!