Delay function for unit tests (#3767)

Sometimes, it is useful it useful to be able to delay a unit test and have
the back async processing running too. This function continuously calls
io_ctx.run_one_for in system class for the specified amount of time.

nano/test_common/system.cpp

@@ -307,6 +307,22 @@ std::error_code nano::system::poll_until_true (std::chrono::nanoseconds deadline
 	return ec;
 }
 
+/**
+ * This function repetitively calls io_ctx.run_one_for until delay number of milliseconds elapse.
+ * It can be used to sleep for a duration in unit tests whilst allowing the background io contexts to continue processing.
+ * @param delay milliseconds of delay
+ */
+void nano::system::delay_ms (std::chrono::milliseconds const & delay)
+{
+	auto now = std::chrono::steady_clock::now ();
+	auto endtime = now + delay;
+	while (now <= endtime)
+	{
+		io_ctx.run_one_for (endtime - now);
+		now = std::chrono::steady_clock::now ();
+	}
+}
+
 namespace
 {
 class traffic_generator : public std::enable_shared_from_this<traffic_generator>

nano/test_common/system.hpp

@@ -43,6 +43,7 @@ public:
 	 */
 	std::error_code poll (std::chrono::nanoseconds const & sleep_time = std::chrono::milliseconds (50));
 	std::error_code poll_until_true (std::chrono::nanoseconds deadline, std::function<bool ()>);
+	void delay_ms (std::chrono::milliseconds const & delay);
 	void stop ();
 	void deadline_set (std::chrono::duration<double, std::nano> const & delta);
 	std::shared_ptr<nano::node> add_node (nano::node_flags = nano::node_flags (), nano::transport::transport_type = nano::transport::transport_type::tcp);