MB-19605: Add more tests for stats

We've added range checking for the calls to snprintf to ensure
that we don't blow the buffer. This patch add a bunch of tests
that call the various stats groups to ensure that they don't
crash or blow the buffer...

Change-Id: I3a16a59c7fad74504a86ff4c9c287b0259d41420
Reviewed-on: http://review.couchbase.org/64330
Reviewed-by: Dave Rigby <daver@couchbase.com>
Well-Formed: buildbot <build@couchbase.com>
Tested-by: buildbot <build@couchbase.com>