tests/intel/gem_exec_balancer: Decrease required load in `full` test

Adjust the minimal load in the `full` test. It's a minor tolerance
issue expected 90% got 86.6%.

Closes: drm/i915/kernel#13003


Signed-off-by: Sebastian Brzezinka <sebastian.brzezinka@intel.com>
Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Krzysztof Karas <krzysztof.karas@intel.com>