Unify leak detection across testing frameworks

[mattsta]

The regular tests do leak detection in test_helper.tcl, but the Sentinel and Cluster tests are managed by instances.tcl which has no leak detection.

I don't see an easy way to incorporate leak detection in instances.tcl, so this is a TODO for us to figure it out eventually. We may even want to consolidate how tests are run. Ideally we should have leak detection before kill_instance and as part of cleanup at the end (or, make a the_end for instances.tcl) too.

Also, the non-main tests (Sentinel, Cluster) sometimes leak processes after failures. Maybe we're missing some pid tracking or error catching so we can kill servers after test failures/crashes?

(The absence of leak detection on non-main tests probably allowed the recent Sentinel leak to go undetected in the wild for a long time.)