Refactor staking test modules into own files

[gpestana]

The staking tests.rs has about 7k LOC. Most of the tests are nicely organised into modules, consider refactor each mod into a new file for readability.

It may be that the module-less tests alone will be lot of LOC, maybe we should consider breaking those down as well on a separate PR. My suggestion on how to proceed are the following:

First iter/PR:

• Open PR refactoring the current tests into it's own mod folder /staking/src/tests/
• Refactor mod tests into their own file. e.g. tests under mod staking_ledger should be in /staking/src/tests/staking_ledger.rs
• Tests without module should end up in /staking/src/tests/mod.rs

Second iter/PR:

• consider re-org tests in mod.rs into relevant submodules

[gpestana]

ftr, @vicenterocha is interested in picking up this issue 🚀

[vicenterocha]

Yes I'll take this one!