tests/perf: make oa-exponents subtest more reliable

We know the OA unit might skip some reports from time to time (reasons
include pressure on memory controller, power management, ...). So
rather than checking that the time between periodic reports is about
the period we asked for, let's verify that the time is always longer
or equal to the period we've asked for.

We still have to leave some room for errors. Here is dump of an error
in this updated test :

(perf:405) DEBUG: report0019 ts=e217de20 hw_id=0x00000014 delta=64
(perf:405) DEBUG: report0020 ts=e217de60 hw_id=0x00000014 delta=64
(perf:405) DEBUG: report0021 ts=e217dea0 hw_id=0x00000014 delta=64
(perf:405) DEBUG: report0022 ts=e217df66 hw_id=0x00000014 delta=198 ******
(perf:405) DEBUG: report0023 ts=e217dfa0 hw_id=0x00000014 delta=58 ******
(perf:405) DEBUG: report0024 ts=e217dfe0 hw_id=0x00000014 delta=64
(perf:405) DEBUG: report0025 ts=e217e020 hw_id=0x00000014 delta=64
(perf:405) DEBUG: report0026 ts=e217e060 hw_id=0x00000014 delta=64

As you can see there is a discrepency in the periodic reports. I have
no explanation for it. This isn't a programming error since the same
context has correct periods before and after, so it must be some kind
of hardware glitch/corner-case that hasn't be been documented.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>

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