Drop bomb rework and testing in tor-memtrack

The following discussion from !2091 (merged) should be addressed:

• @gabi-250 started a discussion: (+2 comments)

  It seems testable if we add some indirection: we could store a callback in Ref that specifies what to do on drop() (in non-test code, it would be a callback to a function that contains the current body of drop).

See the whole thread there for what should be done.