[posix] implement virtual time and update tests to utilize virtual time (#2508)

This commit adds the following features:

- Example posix drivers to support virtual time simulation, allowing tests
  to run faster than real time.

- A python-based simulation driver that coordinates the virtual time
  simulations.  Each alarm event and message transmission across all nodes
  are scheduled in a single discrete time event queue.  As a result, only
  a single node is making forward progress at a time.

- Updated tests to utilize new virtual-time simulator, allowing them to run
  faster than real time.

- To enable virtual time feature, define VIRTUAL_TIME=1 env var and set
  CPPFLAGS=-DOPENTHREAD_POSIX_VIRTUAL_TIME=1.

tests/scripts/thread-cert/node.py

@@ -38,9 +38,11 @@ import unittest
 import config
 
 class Node:
-    def __init__(self, nodeid, is_mtd=False):
+    def __init__(self, nodeid, is_mtd=False, simulator=None):
+        self.simulator = simulator
+
         if sys.platform != 'win32':
-            self.interface = node_cli.otCli(nodeid, is_mtd)
+            self.interface = node_cli.otCli(nodeid, is_mtd, simulator=simulator)
         else:
             self.interface = node_api.otApi(nodeid)
 
@@ -239,7 +241,7 @@ class Node:
     def scan(self):
         return self.interface.scan()
 
-    def ping(self, ipaddr, num_responses=1, size=None, timeout=5000):
+    def ping(self, ipaddr, num_responses=1, size=None, timeout=5):
         return self.interface.ping(ipaddr, num_responses, size, timeout)
 
     def reset(self):