tests: sensor: generic: Set thread priorities for back-to-back readings

In order to support back-to-back readings, it's required that the
RTIO workq threads priority is higher than the caller issuing the
back-to-back requests, otherwise the requests will fail as P4WQ won't
have the opportunity to clear the is_done semaphore, and the subsequent
request will be rejected.

Signed-off-by: Luis Ubieda <luisf@croxel.com>