Investigate ep_testsuite not catching races under TSAN

A family of races (MB-48816) have been caught with TSAN-build cluster_run, but were not caught by relevant ep_testsuite tests.

Stat groups: "dcp", "dcpagg", and "checkpoint" previously wrote responses with add_stat from a background task. This has been identifed as racy by TSAN in a cluster_run in MB-48816.

ep_testsuite and ep_testsuite_dcp tests exercise all of these groups, but did not report races despite them being present for a reasonable amount of time.

Investigate whether this is a flaw in ep_testsuite which may mask other races (e.g., MockCookie differs from Cookie in that it has a mutex, and some operations lock it), or if the tests simply do not exercise the relevant code paths, and need to be improved.