Feat: invariant tests

[Jean-Grimal]

I implemented 4 contexts :

• 3 with fix prices (and therefore no liquidations) : one with one market and only one user, one with one market and 8 users, and one with 2 markets and 8 users.
• Another one with variable prices (variations are bounded between 2 and 20%), with one market and 8 users

I wanted to cover lots of scenarios, but maybe one of this context is useless (the one with a single user for exemple), and should be removed

[Rubilmax]

I wanted to cover lots of scenarios, but maybe one of this context is useless (the one with a single user for exemple), and should be removed

I agree the single market, single user test case is not that interesting

[MathisGD]

great job !