Add unit tests for linear and nonlinear API

[tmigot]

From NLPModels.jl 0.18 we have access to the linear constraints.
We will need to add unit tests for the linear and nonlinear API.

• Add linear API in test problems
• Add tests in check-dimensions.jl
• Add tests in multi-precision.jl
• Add tests in coord-memory.jl
• Add tests in consistency.jl in the function consistent_counters
• Add linear API for quasi-Newton models (NLPModelsModifiers.jl)
• Add linear API for slack models (NLPModelsModifiers.jl)
• Add line API for FeasibilityFormNLS models (NLPModelsModifiers.jl)
• Add tests in consistency.jl in the function consistent_functions

[tmigot]

I gave up on the Add line API for FeasibilityFormNLS models (NLPModelsModifiers.jl), because it would require access to the linear part of the residual. So, I suggest we make an issue for this on NLPModelsModifiers.jl for later, and skip this in the consistency checks in NLPModelsTest.

[abelsiqueira]

I agree. Can you create the issue and close this afterwards?