[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/config.py

@@ -37,6 +37,7 @@ import mle
 import net_crypto
 import network_data
 import network_layer
+import simulator
 import sniffer
 
 MESH_LOCAL_PREFIX = 'fdde:ad00:beef::/64'
@@ -67,6 +68,8 @@ INFINITE_COST_TIMEOUT = 90
 MAX_ADVERTISEMENT_INTERVAL = 32
 MLE_END_DEVICE_TIMEOUT = 100
 
+VIRTUAL_TIME = bool(os.getenv('VIRTUAL_TIME', False))
+
 def create_default_network_data_prefix_sub_tlvs_factories():
     return {
         network_data.TlvType.HAS_ROUTE: network_data.HasRouteFactory(
@@ -331,3 +334,9 @@ def create_default_thread_message_factory(master_key=DEFAULT_MASTER_KEY):
 
 def create_default_thread_sniffer(nodeid=SNIFFER_ID):
     return sniffer.Sniffer(nodeid, create_default_thread_message_factory())
+
+
+def create_default_simulator():
+    if VIRTUAL_TIME:
+        return simulator.VirtualTime()
+    return simulator.RealTime()