Fix tests adding multiple variable bounds

[odow]

To fix orderedindices test, we use different variables.
Closes #403

To fix the basic_constraint_tests, we skip the test that addconstraints! works for SingleVariable-in-Set constraints.
Closes #404

[codecov-io]

Codecov Report

Merging #434 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #434      +/-   ##
==========================================
+ Coverage   95.68%   95.69%   +<.01%     
==========================================
  Files          40       40              
  Lines        5242     5244       +2     
==========================================
+ Hits         5016     5018       +2     
  Misses        226      226

Impacted Files	Coverage Δ
src/Test/UnitTests/basic_constraint_tests.jl	98.33% <100%> (ø)	⬆️
src/Test/modellike.jl	100% <100%> (ø)	⬆️

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