Fix and Test

[Sid-Bhatia-0]

1. Bug fixes
   1. Fix SequentialRooms: Create and use the big world on each reset!
   2. Fix DynamicObstacles: Use agent_start_pos instead of get_agent_pos(env) to check while placing obstacles. This is because the agent pos is reset in the end of reset! method, and so get_agent_pos(env) would return the agent pos from the previous run.
2. Add some useful tests and remove unnecessary tests. Also, combined RLBase testset so that now there is only one testset corresponding to each environment.

[codecov-io]

Codecov Report

Merging #91 (9722848) into master (ed6a1d4) will decrease coverage by 1.15%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
- Coverage   78.32%   77.16%   -1.16%     
==========================================
  Files          15       15              
  Lines         655      657       +2     
==========================================
- Hits          513      507       -6     
- Misses        142      150       +8     

Impacted Files	Coverage Δ
src/envs/dynamicobstacles.jl	100.00% <100.00%> (ø)
src/envs/sequentialrooms.jl	99.18% <100.00%> (+<0.01%)	:arrow_up:
src/grid_world_base.jl	66.00% <0.00%> (-12.00%)	:arrow_down:
src/abstract_grid_world.jl	62.68% <0.00%> (-2.99%)	:arrow_down:

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