Tests for agent static

Add tests that show that agent static is populated always.

Currently we get the following error when env.reset(False,False,False) is called:

Error :  'NoneType' object has no attribute '__dict__'
Traceback (most recent call last):
  File "/Users/mohanty/work/SBB/flatland/flatland/flatland/evaluators/service.py", line 621, in run
    self.handle_env_create(command)
  File "/Users/mohanty/work/SBB/flatland/flatland/flatland/evaluators/service.py", line 359, in handle_env_create
    random_seed=RANDOM_SEED
  File "/Users/mohanty/work/SBB/flatland/flatland/flatland/envs/rail_env.py", line 320, in reset
    self.restart_agents()
  File "/Users/mohanty/work/SBB/flatland/flatland/flatland/envs/rail_env.py", line 250, in restart_agents
    self.agents = EnvAgent.list_from_static(self.agents_static)
  File "/Users/mohanty/work/SBB/flatland/flatland/flatland/envs/agent_utils.py", line 124, in list_from_static
    for handle, oEAS in zip(handles, lEnvAgentStatic)]
  File "/Users/mohanty/work/SBB/flatland/flatland/flatland/envs/agent_utils.py", line 124, in <listcomp>
    for handle, oEAS in zip(handles, lEnvAgentStatic)]
AttributeError: 'NoneType' object has no attribute '__dict__'

relates to #251 (closed)