[Test] Add test_variable.jl

[odow]

Part of #1398

[odow]

@blegat I think this is easier than defining every condition that you need to pass the test. Why not just trap UnsupportedConstraint and UnsupportedAttribute errors?

[odow]

So this works nicely for GLPK

julia> MOI.Test.runtests(model, config, warn_unsupported = true)
Test Summary:              | Pass  Total
test_variable_VariableName |    4      4
Test Summary:              | Pass  Total
test_variable_add_variable |    2      2
Test Summary:               | Pass  Total
test_variable_add_variables |    2      2
Test Summary:        | Pass  Total
test_variable_delete |    3      3
┌ Warning: Skipping test_variable_delete_Nonnegatives: RequirementUnmet: MOI.supports_add_constrained_variables(model, MOI.Nonnegatives)
└ @ MathOptInterface.Test ~/.julia/dev/MathOptInterface/src/Test/Test.jl:247
Test Summary:                     |
test_variable_delete_Nonnegatives | No tests
┌ Warning: Skipping test_variable_delete_Nonnegatives_row: RequirementUnmet: MOI.supports_add_constrained_variables(model, MOI.Nonnegatives)
└ @ MathOptInterface.Test ~/.julia/dev/MathOptInterface/src/Test/Test.jl:247
Test Summary:                         |
test_variable_delete_Nonnegatives_row | No tests
┌ Warning: Skipping test_variable_delete_SecondOrderCone: RequirementUnmet: MOI.supports_add_constrained_variables(model, MOI.SecondOrderCone)
└ @ MathOptInterface.Test ~/.julia/dev/MathOptInterface/src/Test/Test.jl:247
Test Summary:                        |
test_variable_delete_SecondOrderCone | No tests
Test Summary:                  | Pass  Total
test_variable_delete_variables |    9      9
Test Summary:                   | Pass  Total
test_variable_get_VariableIndex |    2      2
Test Summary:                                | Pass  Total
test_variable_solve_Integer_with_lower_bound |    4      4
Test Summary:                                | Pass  Total
test_variable_solve_Integer_with_upper_bound |    4      4
Test Summary:                                  | Pass  Total
test_variable_solve_ZeroOne_with_0_upper_bound |    4      4
Test Summary:                                | Pass  Total
test_variable_solve_ZeroOne_with_upper_bound |    4      4
Test Summary:                       | Pass  Total
test_variable_solve_with_lowerbound |   10     10
Test Summary:                       | Pass  Total
test_variable_solve_with_upperbound |   12     12

@blegat I wonder if we even need that excluded_attributes thing we talked about.