Clean up tests

[exaexa]

• replace GLPK by Tulip
• remove Clp
• prepare the Distributed workers in advance (saves time if more distributed stuff is tested, which is the case now)

[codecov]

Codecov Report

Merging #277 (96cc9bb) into master (cf53296) will increase coverage by 1.86%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #277      +/-   ##
==========================================
+ Coverage   87.60%   89.47%   +1.86%     
==========================================
  Files          49       49              
  Lines        1138     1150      +12     
==========================================
+ Hits          997     1029      +32     
+ Misses        141      121      -20     

Impacted Files	Coverage Δ
src/banner.jl	0.00% <ø> (ø)
src/base/types/Serialized.jl	80.64% <83.33%> (+64.51%)	⬆️
src/base/types/CoreModel.jl	100.00% <0.00%> (ø)
src/base/types/abstract/MetabolicModel.jl	95.83% <0.00%> (+1.09%)	⬆️

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