tests/perf: prevent power management to kick in when necessary

Some of our tests measure that the OA unit produces reports at
expected time intervals (as configured through the PERF_OPEN
ioctl). It turns out the power management plays a role in the decision
of the OA unit to write reports to memory. Under normal circumstances
we don't really mind if the unit misses one report here or there, but
for our tests it makes pretty difficult to verify whether we've made a
mistake in the configuration.

To work around this, let's prevent power management to kick in by
holding /dev/cpu_dma_latency opened for the following tests :

  - enable-disable
  - blocking
  - polling
  - buffer-fill
  - oa-exponents

Many thanks to Chris Wilson for suggesting this!

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

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