Refactor test/Containers into functional form

[odow]

No description provided.

[odow]

These test failures are precisely the reason to refactor the tests. Changing the scope of files in test/Containers breaks an unrelated test in test/containers.jl (because test/Containers no longer includes a global using JuMP statement).

[codecov]

Codecov Report

Base: 97.69% // Head: 97.69% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (82ba59e) compared to base (8f6081b).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3158   +/-   ##
=======================================
  Coverage   97.69%   97.69%           
=======================================
  Files          33       33           
  Lines        4470     4472    +2     
=======================================
+ Hits         4367     4369    +2     
  Misses        103      103           

Impacted Files	Coverage Δ
src/JuMP.jl	95.83% <0.00%> (+0.03%)	⬆️

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