Update steps_limit then just run some additional steps

Return the best solution so far (if any)

Property definitions

ai $ SearchSolver :: run_steps
	# Update `steps_limit` then just run some additional steps
	# Return the best solution so far (if any)
	fun run_steps(steps: Int): nullable SearchNode[S,A]
	do
		assert steps > 0
		self.steps_limit = self.steps + steps
		return run
	end
lib/ai/search.nit:364,2--371,4