Reduce default poll time from 50ms to 10ms

Some testcases (e.g. node.fork_publish) are racy because too many things can happen between 2 polls. While this doesn't fix the racy testcase, it greatly reduces the likelyhood that testcases are failing due to timing issues

nano/test_common/system.hpp

@@ -49,10 +49,10 @@ namespace test
 		/** Generate work with difficulty between \p min_difficulty_a (inclusive) and \p max_difficulty_a (exclusive) */
 		uint64_t work_generate_limited (nano::block_hash const & root_a, uint64_t min_difficulty_a, uint64_t max_difficulty_a);
 		/**
-		 * Polls, sleep if there's no work to be done (default 50ms), then check the deadline
+		 * Polls, sleep if there's no work to be done (default 10ms), then check the deadline
 		 * @returns 0 or nano::deadline_expired
 		 */
-		std::error_code poll (std::chrono::nanoseconds const & sleep_time = std::chrono::milliseconds (50));
+		std::error_code poll (std::chrono::nanoseconds const & sleep_time = std::chrono::milliseconds (10));
 		std::error_code poll_until_true (std::chrono::nanoseconds deadline, std::function<bool ()>);
 		void delay_ms (std::chrono::milliseconds const & delay);
 		void deadline_set (std::chrono::duration<double, std::nano> const & delta);