tests: make RA tests run on VPP time

the IPv6 RA tests take timing into the account,
but the time inside VPP may go slightly differently compared
to the time inside the driving python thread,
if the machine running the tests is heavily loaded.
Make a sleep function which sleeps "on VPP time" and use it.

Change-Id: I3b34b0164f6e0ec7a619b92ee308089a4a8935e3
Type: test
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>

test/framework.py

@@ -753,6 +753,20 @@ class VppTestCase(unittest.TestCase):
         # add to the list of captures with current timestamp
         cls._captures.append((time.time(), cap_name))
 
+    @classmethod
+    def get_vpp_time(cls):
+        return float(cls.vapi.cli('show clock').replace("Time now ", ""))
+
+    @classmethod
+    def sleep_on_vpp_time(cls, sec):
+        """ Sleep according to time in VPP world """
+        # On a busy system with many processes
+        # we might end up with VPP time being slower than real world
+        # So take that into account when waiting for VPP to do something
+        start_time = cls.get_vpp_time()
+        while cls.get_vpp_time() - start_time < sec:
+            cls.sleep(0.1)
+
     @classmethod
     def pg_start(cls):
         """ Enable the PG, wait till it is done, then clean up """