P-chain Add UTs around stakers persistence in platformvm state

[abi87]

Why this should be merged

These are the UTs I wish I had before venturing in refactoring writeCurrentStakers in #2074 and following
Whenever a staker is added or deleted a whole bunch of data structures are updated and possibly stored.
We don't currently assert the behaviour of these data structures. The added UTs cover the gap.

This should have been the very first change I made (crate the test you wish you had).
I didn't and I had other review on the PRs that actually change to code.
So I didnt' rebase the prod-code changing PRs over this testing PR.
I only listed them under the same issue #2512

How this works

There a 16 combinations. A staker can be:

• validator or delegator
• primary network or subnet one
• current or pending
• being added or removed

For each combination the PR set the state and checks that the ancillary data structures are update correctly.
Moreover it rebuild the state from the db and carries out the very same checks, to make sure that state is persisted correctly.

How this was tested

Just add UTs

[danlaine]

I think this is an improvement over what currently exists, so I'm approving this, but I do have some reservations about the readability of these tests. I think that's more of a reflection of the platformvm/state package than anything else. Right now, I think there are a lot of implicit invariants about how *state is called, which are checked in tests, which makes reasoning more difficult.

For example, we do something like this in a bunch of places:

				s.PutCurrentValidator(staker)
				s.AddTx(addPermValTx, status.Committed) // this is currently needed to reload the staker

Now, it isn't explicitly mentioned anywhere that you need to call AddTx along with PutCurrentValidator, but we implicitly expect that usage and use it in the test. Just an example of what I mean.

Anyway, other than my comment about the test naming duplicates, I think this is alright.

[dhrubabasu]

I share the same concerns as @danlaine... state's hidden invariants are tough to reason about

Thanks for adding these tests ❤️