Add aggregate metric test scenario with buckets > 1

[vnugent]

Be able to generate expected aggregate data to handle multiple buckets. For example,

raw data
[ {time1, data}, {time2, data}, {time3, data}, {time4, data}]
test
get aggregate metric start=time1, end=time4, buckets=2

expected result
[{start:time1, end:time2, min, max, avg... of 1st and 2nd pair}, {start:time3, end:time4, min, max of 3rd and 4th pair}]