Unit tests for the condition number of a problem

[Stéphane Caron]

@aescande Would you have some meaningful unit tests at the ready for the condition number of a problem with active set #232? I think we will want to check at least the following cases:

• No constraint: done ✔️
• Only equality constraints: ❔
• Active linear inequalities: ❔
• Active lower bounds: ❔
• Active upper bounds: ❔

Ideally we would have one problem for each, and the corresponding condition number.