Test getters of func/set for flip_sign_bridge

[blegat]

See #750

[codecov-io]

Codecov Report

Merging #755 into master will increase coverage by 0.05%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master     #755      +/-   ##
==========================================
+ Coverage   94.32%   94.38%   +0.05%     
==========================================
  Files          56       56              
  Lines        6220     6478     +258     
==========================================
+ Hits         5867     6114     +247     
- Misses        353      364      +11

Impacted Files	Coverage Δ
src/Bridges/flip_sign_bridge.jl	95.65% <92.85%> (+0.53%)	⬆️
src/Bridges/lazybridgeoptimizer.jl	96.55% <0%> (-0.89%)	⬇️
src/Utilities/functions.jl	94.97% <0%> (-0.05%)	⬇️
src/Test/UnitTests/basic_constraint_tests.jl	98.61% <0%> (+0.61%)	⬆️
src/Bridges/slackbridge.jl	95% <0%> (+1.45%)	⬆️

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