Improve MOI test usage

[rschwarz]

• following discussion in Add tests, extend docstring for ScalarQuadraticFunction. jump-dev/MathOptInterface.jl#697,
• use MOIB.full_bridge_optimizer for the MOI testsets,
• call individual tests (not testsets with exclusions) when using direct Optimizer.

Still need to exlude tests for a variety of reasons (can't query vars by name, SCIP's SOC constraints make strong assumptions about var bounds, can't delete vars from constraints), but feel more future-proof w.r.t. to new tests being added to MOI 😄

[codecov]

Codecov Report

Merging #111 into master will increase coverage by 0.21%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #111      +/-   ##
==========================================
+ Coverage   13.11%   13.32%   +0.21%     
==========================================
  Files         133      133              
  Lines        4217     4217              
==========================================
+ Hits          553      562       +9     
+ Misses       3664     3655       -9

Impacted Files	Coverage Δ
src/MOI_wrapper/variable.jl	77.19% <0%> (ø)	⬆️
src/wrapper/cons_linear.jl	45.45% <0%> (+13.63%)	⬆️
src/MOI_wrapper/linear_constraints.jl	80.48% <0%> (+14.63%)	⬆️

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