performs_within()

Just found pgTAP and it's AWESOME.  Thanks for the great work.

I wanted a benchmark test that was a little more robust, so I created performs_within based off the performs_ok function that already exists.

performs_within() will run a query multiple times, recording how long the query took each time.  It will then throw out the top and bottom 10% of those runs and grab the average of the remaining middle 80%.  It will then compare that average to an expected average.  If the average is within a range around the expected average, the test passes.

I hope you find it useful.

Thanks again,
Ryan