[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/Cert_6_3_02_NetworkDataUpdate.py

@@ -30,6 +30,7 @@
 import time
 import unittest
 
+import config
 import node
 
 LEADER = 1
@@ -37,9 +38,11 @@ ED = 2
 
 class Cert_5_6_2_NetworkDataUpdate(unittest.TestCase):
     def setUp(self):
+        self.simulator = config.create_default_simulator()
+
         self.nodes = {}
         for i in range(1,3):
-            self.nodes[i] = node.Node(i, (i == ED))
+            self.nodes[i] = node.Node(i, (i == ED), simulator=self.simulator)
 
         self.nodes[LEADER].set_panid(0xface)
         self.nodes[LEADER].set_mode('rsdn')
@@ -56,19 +59,20 @@ class Cert_5_6_2_NetworkDataUpdate(unittest.TestCase):
         for node in list(self.nodes.values()):
             node.stop()
         del self.nodes
+        del self.simulator
 
     def test(self):
         self.nodes[LEADER].start()
-        self.nodes[LEADER].set_state('leader')
+        self.simulator.go(5)
         self.assertEqual(self.nodes[LEADER].get_state(), 'leader')
 
         self.nodes[ED].start()
-        time.sleep(5)
+        self.simulator.go(5)
         self.assertEqual(self.nodes[ED].get_state(), 'child')
 
         self.nodes[LEADER].add_prefix('2001:2:0:1::/64', 'paros')
         self.nodes[LEADER].register_netdata()
-        time.sleep(5)
+        self.simulator.go(5)
 
         addrs = self.nodes[ED].get_addrs()
         self.assertTrue(any('2001:2:0:1' in addr[0:10] for addr in addrs))
@@ -81,11 +85,11 @@ class Cert_5_6_2_NetworkDataUpdate(unittest.TestCase):
 
         self.nodes[LEADER].add_prefix('2001:2:0:2::/64', 'paros')
         self.nodes[LEADER].register_netdata()
-        time.sleep(5)
+        self.simulator.go(5)
 
         self.nodes[LEADER].add_whitelist(self.nodes[ED].get_addr64())
         self.nodes[ED].add_whitelist(self.nodes[LEADER].get_addr64())
-        time.sleep(10)
+        self.simulator.go(10)
 
         addrs = self.nodes[ED].get_addrs()
         self.assertTrue(any('2001:2:0:1' in addr[0:10] for addr in addrs))