Use gevent sleep for switching context

Was changed `time.sleep` to `gevent.sleep` to allow current greenlet to
sleep and others to run. If it uses time.sleep() greenlet's context is
not changed from the main process to test greenlet. As a result, there
was no data received by the main process during hanging tarantool and
the suite was fallen down by common timeout (no output timeout). Using
greenlet timeout allows to fall down by test timeout.

Fixes: #276

lib/tarantool_server.py

@@ -449,7 +449,7 @@ def seek_wait(self, msg, proc=None, name=None):
         while True:
             if os.path.exists(self.path):
                 break
-            time.sleep(0.001)
+            gevent.sleep(0.001)
 
         with open(self.path, 'r') as f:
             f.seek(self.log_begin, os.SEEK_SET)
@@ -460,7 +460,7 @@ def seek_wait(self, msg, proc=None, name=None):
                         raise TarantoolStartError(name)
                 log_str = f.readline()
                 if not log_str:
-                    time.sleep(0.001)
+                    gevent.sleep(0.001)
                     f.seek(cur_pos, os.SEEK_SET)
                     continue
                 if re.findall(msg, log_str):