qa/1064: accommodate integer rounding in pmConvScale()
qa/1064: accommodate integer rounding in pmConvScale()

Previously, this test would fail 1 in 1024 times it is run
(on average).

Finally tracked the cause down.
- sample.byte_rate is indeed in the range 0..1023
- the derived metric rescale() function calls pmConvScale()
- and pmConvScale() does integer rounding when the result needs to
  be an integer value (as it does here)

So the maximum expected value is 60, not 1023*60/1024.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp