Fix flaky integration test TestDynamicNamespaceDelete

[richardartoul]

The test was flaky because it was relying upon the behavior the Snapshot() functionality of the tally library.

The Snapshot() function in tally will return counter values, but only those that occurred since the last flush/report.

As a result, the test would sometimes fail due to the following race:

1. Dynamic namespace watcher receives new value from KV
2. Namespace watcher calls Inc(1) on "invalid-update" metric
3. Tally "Scope" object reports metrics to reporter
4. We called Snapshot() to check if the counter has incremented yet, but it returns 0 indefinitely because the metric has already been reported and is thus unavailable via the Snapshot() call

This PR fixes the race by relying on the TestStatsReporter instead of Snapshot(). Since the TestStatsReporter receives every report, it can keep track of every metric and the race is no longer an issue.

[prateek]

LGTM

[coveralls]

Coverage decreased (-0.02%) to 78.319% when pulling 88c940e on richardartoul:ra_debug_namespace_delete into a2c7092 on m3db:master.