QStack: Add a simple benchmark.

This covers the only real additions over QVector: push and pop. Really, there
isn't too much specific to benchmark here, but we're interested in one specific
case: that of pushing and popping a single item repeatedly.

With the current QVector behavior, this causes constant deallocation, which
makes it morbidly slow. This behavior will be reviewed in a subsequent commit.

Results (not that anyone really cares) for me:
    PASS   : tst_QStack::qstack_push()
    RESULT : tst_QStack::qstack_push():
         1.9 msecs per iteration (total: 61, iterations: 32)
    PASS   : tst_QStack::qstack_pop()
    RESULT : tst_QStack::qstack_pop():
         8.2 msecs per iteration (total: 66, iterations: 8)
    PASS   : tst_QStack::qstack_pushpopone()
    RESULT : tst_QStack::qstack_pushpopone():
         80 msecs per iteration (total: 80, iterations: 1)

Change-Id: I3530888abbfcfcef39318d6be6d5b07306a4704e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>