test: create pool

[rflechtner]

re/ KILTProtocol/ticket#3671

Adds unit tests for creating pools.
To be rebased and merged after #780

Tests currently failing (fix TBD in another PR):

• Exhausting Asset Id value space should result in ArithmeticError::Overflow
• You should not be able to create a pool where a bonded currency is also the pool's collateral

Checklist:

• I have verified that the code works
  • No panics! (checked arithmetic ops, no indexing array[3] use get(3), ...)
• I have verified that the code is easy to understand
  • If not, I have left a well-balanced amount of inline comments
• I have left the code in a better state
• I have documented the changes (where applicable)
  • Either PR or Ticket to update the Docs
  • Link the PR/Ticket here

[Ad96el]

Maybe move the balance to mock. It is used multiple times

[rflechtner]

e458d8c

[Ad96el]

Can we also check the deposit? We have checked it for a single currency. For multiple, it should differ.

[rflechtner]

I can't seem to be able to link to the right line, but this is being checked with

			assert_eq!(
 				Balances::free_balance(ACCOUNT_00),
 				initial_balance.sub(BondingPallet::calculate_pool_deposit(3))
 			);

[ntn-x2]

Good enough for me 👍