IMPALA-8007: Fix test_slow_subscriber
IMPALA-8007: Fix test_slow_subscriber

Previously, test_slow_subscriber verified a slow subscriber by
checking for a precise time window since the last heartbeat.
This was non-deterministic since sleep duration in python depends
on the machine's workload. This change makes it deterministic by
checking for an increase in the time since last heartbeat.

Testing: Executed the test 1000 times in a loop.

Change-Id: Ibeed543a145076cd11d5d0e441a257111a66497d
Reviewed-on: http://gerrit.cloudera.org:8080/12216
Reviewed-by: Impala Public Jenkins <039b6b20426ff68ebe492b3021890d8c8fd010c9@cloudera.com>
Tested-by: Impala Public Jenkins <039b6b20426ff68ebe492b3021890d8c8fd010c9@cloudera.com>

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