Refactor staking test modules into their own files

[vicenterocha]

This PR refactors substrate staking tests, as detailed in #3178.

The idea is to do this in 2 PRs. On this one:

1. Move all mod tests from tests.rs into their own test file under a new tests/ folder
2. Move all non mod tests from tests.rs into tests/mod.rs

Even we this changes the tests/mod.rs is still quiet big (6k LOC). Next PR should address it by extrating tests/mods.rs into further submodules.

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[gpestana]

@vicenterocha thanks for the PR! One quick comment, we try to avoid rewriting history/force push on intermediate commits on open PRs -- especially on PRs ready to review, which is not the case here but it's still better to not squash int commits in general. The commits will be squashed before merging so no problem with adding a bunch of commits to the PR.

[gpestana]

LGTM, thanks!

regarding the failing jobs, there was a recent fix to polkavm that should fix the build-runtimes-polkavm job -- could you merge the most recent master into your branch?

[Ank4n]

@vicenterocha thanks for the contribution. Quick question, when I run these tests (cargo test -p pallet-staking) I see one less test being ran than the previous count (Old: 156, New: 155). Was there a test you removed or?

[vicenterocha]

@Ank4n thanks for having a look. Running tests on this branch actually gives me 156, whereas on master it gives me 155.

refactor-staking-tests:

master:

I've compare both runs and it seems like this is the the additional test:
test mock::test_genesis_config_builds ... ok

I can provide a full test output of both master and current branch.

Not sure exactly how this test is defined though.

[Ank4n]

@Ank4n thanks for having a look. Running tests on this branch actually gives me 156, whereas on master it gives me 155.

refactor-staking-tests:

master:

I've compare both runs and it seems like this is the the additional test: test mock::test_genesis_config_builds ... ok

I can provide a full test output of both master and current branch.

Not sure exactly how this test is defined though.

Thanks for investigating it. The test was added couple of weeks ago. My guess is I ran the test with your branch that was not updated with master which is why I missed one. And vice-versa for you, your master might be outdated.

[vicenterocha]

Running on this branch

Running on master

[gpestana]

@vicenterocha thanks a lot for helping with this work, we have a bunch of changes coming up related to the pallet-staking with the multi-block election work and I think it's better to refactor all the tests during that work. In any case, I can point you some other issues to help with if you want. Thanks :)