added test to load msgpack env and add agent_static

tests/test_env_edit.py

+
+from flatland.envs.rail_env import RailEnv
+# from flatland.envs.agent_utils import EnvAgent
+from flatland.envs.agent_utils import EnvAgentStatic
+
+
+def test_load_env():
+    env = RailEnv(10, 10)
+    env.load("env-data/tests/test-10x10.mpk")
+
+    agent_static = EnvAgentStatic((0, 0), 2, (5, 5)) 
+    env.add_agent_static(agent_static)
+    assert env.get_num_agents() == 1
+
+