Create test for initial chunk loading

[DaveyBiggers]

As #195 illustrated, there can be issues around the start of the mission. #327 should have fixed this, but it would be good to have some sort of test.
This could generate a random mission each run which:

• Chooses a random start point for the agent
• Draws a random set of blocks there
• Catches the grid observations

The test would then check the observations at the start of the mission.

[DaveyBiggers]

Fixed in 0543d42