Refactor tests

[blegat]

The idea is to be able to run subsets of the tests easily.
So if you just want to test utilities, you just do

$ julia test/Utilities/Utilities.jl

and if you just want to test the caching optimizer, you can do

$ julia test/Utilities/caching_optimizer.jl

The second important aspect is that we would like to have several testsets when we do that so that we have information about the progress of the tests.
If it runs for a few seconds and then just says

$ julia test/Utilities/caching_optimizer.jl
Test Summary:     | Pass  Total
Caching Optimizer |  109    109

then it's not helpful.
For this reason, inside test/Utilities/caching_optimizer.jl, there is a few test sets and in test/Utilities/Utilities.jl, we have

@testset "Caching Optimizer" begin
    include("caching_optimizer.jl")
end

so that if we run test/Utilities/caching_optimizer.jl we see each sub-testsets and if we run the whole test/Utilities/Utilities.jl, we just get Caching Optimizer and if we run the whole runtests.jl, we just have Utilities.

[blegat]

Failing on Julia v0.7 when it redefines a struct:
https://github.com/JuliaOpt/MathOptInterface.jl/blob/208a9aa3d579d2811b1e7b6947fcb2b1ca890984/src/Utilities/model.jl#L582
I don't feel it's worth it adding a hack for Julia v0.7 given that it support was dropped:
https://discourse.julialang.org/t/julia-0-6-and-0-7-are-now-officially-unmaintained/20449/2

[codecov-io]

Codecov Report

Merging #715 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #715   +/-   ##
=======================================
  Coverage   93.73%   93.73%           
=======================================
  Files          54       54           
  Lines        5572     5572           
=======================================
  Hits         5223     5223           
  Misses        349      349

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