Stochastic test support plural SampleValueSeries.

After SampleValue was refactored into SampleValueSeries, which
involves plural values under a common super key, the stochastic
test was never refreshed to reflect this reality.  We had other
tests that validated the functionality, but this one was
insufficently forward-ported.
Merge pull request #165 from prometheus/fix/testing/stochastic

Stochastic test support plural SampleValueSeries.