tests: continuous_data_consumer_test: reduce runtime
tests: continuous_data_consumer_test: reduce runtime

continuous_data_consumer_test takes an unreasonable amount of
time to run, especially in debug mode.  Reduce the run time by
reducing the number of loops.
Message-Id: <e44c5766a64be9ad558c2f38b055f0933ea47953@scylladb.com>

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