Disable the singletonthreadpool test cleanup no gc

Disable the singletonthreadpool test cleanup no gc

This test often fails on CI under load and reveals a small
race condition within SingleonThreadPool's "cleanup" system.
As this pool's "cleanup" system is pretty much never used in
any case since this pool is no longer used for SQLite file
databases, this race should have little impact, especially
since the occasional non-closed connection will get closed
out when it's GC'ed in any case.

Change-Id: I68aefc0f9bbfc708c95af0497844e65366fc8429