tests/i915/pxp: test_pxp_stale_buf_optout_execution requires arb session

Subtest 'test_pxp_stale_buf_optout_execution' when run on its own,
could be executing without the PXP Arb session being alive. This
would result in the termination trigger to fail (when writing to
the pxp termination debugfs).

Create a background protected context to start the Arb session so
the simulated termination will succeed.

   v2: - use create_ctx_with_params instead of prepare_exec_assets
         for the background pxp context (Daniele)

Signed-off-by: Alan Previn <alan.previn.teres.alexis@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>