tests/chamelium: Catch and flush hotplug uevents after each plug

This adds calls to igt_hotplug_detected and igt_flush_hotplugs to catch
and flush hotplugs from connector unplug (due to chamelium reset) and
plug. These need to be intercepted so that they are not delayed and
issued after resume, providing a false positive for the test result.

In addition, the final hotplug uevent flush is brought closer to the
suspend call, to decrease the likeliness of false positive.

However, false positives still do happen, because it is not possible to
make sure that the uevent caused by each connector's state change was
caught instead of being delayed and issued at resume time.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@linux.intel.com>
Reviewed-by: Lyude <lyude@redhat.com>