Refactor test/mutable_arithmetics.jl

[odow]

In addition, disable JuMPExtension tests. They double our testing time
without adding much in the way of extra coverage.

Extracted from #2277

[codecov]

Codecov Report

Merging #2284 into master will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2284      +/-   ##
==========================================
+ Coverage   91.25%   91.35%   +0.09%     
==========================================
  Files          42       42              
  Lines        4233     4234       +1     
==========================================
+ Hits         3863     3868       +5     
+ Misses        370      366       -4     

Impacted Files	Coverage Δ
src/constraints.jl	91.11% <0.00%> (-0.75%)	⬇️
src/operators.jl	91.15% <0.00%> (+3.48%)	⬆️

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[odow]

@blegat are you okay dropping the JuMPExtension tests? Saves us nearly 2 minutes of test time.

[blegat]

It's important to test that we use AbstractVariableRef here: https://github.com/jump-dev/JuMP.jl/blob/master/src/mutable_arithmetics.jl#L21
for instance. Maybe we could run them for promote_operation tests only.