[v3] Refined test instrument for container tests

[Benjamin Kosnik]

> I moved my countable class out of list_modifiers.cc and into its own 
> header so I can reuse it in additional testsuite entries.  

Which other testsuite entries would you like to use it with?
Something like this is probably a good idea, to reduce the duplication in 
some of the testsuite code.

> I figured the countable class, renamed from "T" to "CountingObject," 
> deserved its own header rather than adding it to testsuite_hooks.h 
> since it's use is limited to only a very few test cases.  This may be 
> open to debate, but it looks like a static library for testsuite 
> support may be in the offing anyways.

What do you think about a testsuite/util, or some sub-directory differently 
named, for this and the timer code?