Refactor tests, sort environment registrations

[AdamGleave]

No description provided.

[codecov]

Codecov Report

Merging #31 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #31   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         8    +2     
  Lines          228       231    +3     
=========================================
+ Hits           228       231    +3     

Impacted Files	Coverage Δ
tests/test_envs.py	100.00% <ø> (ø)
tests/test_mujoco_rl.py	100.00% <ø> (ø)
tests/test_base_env.py	100.00% <100.00%> (ø)
tests/test_diagnostics.py	100.00% <100.00%> (ø)

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[pedrofreire]

LGTM :)