tests/i915/gem_exec_fence: Configure correct context

The update to use intel_ctx_t missed a line that configures the
context to allow hanging. Fix that.

Fixes: 09c36188 ("tests/i915/gem_exec_fence: Convert to intel_ctx_t (v2)")
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>