perf test: Make metric testing more robust

When testing metric expressions we fake counter values from 1 going
upward. For some metrics this can yield negative values that are clipped
to zero, and then cause divide by zero failures.

Such clipping is questionable but may be a result of tools automatically
generating metrics. A workaround for this case is to try a second time
with counter values going in the opposite direction.

This case was seen in a metric like:
  event1 / max(event2 - event3, 0)
But it may also happen in more sensible metrics like:
  event1 / (event2 + event3 - 1 - event4)

Reviewed-by: John Garry <[email protected]>
Signed-off-by: Ian Rogers <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kajol Jain <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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