Separated fuzz tests, added pool_creation

[Chralt98]

Related to [Swaps] Improve fuzz tests #290

The approach at the moment is to fuzz test each dispatchable of the swaps pallet with random and valid pool id's.

The challenge is to check multiple combinations of valid pool id's and these dispatchables. Therefore it would be wasted time to check and find invalid pool id's.

[Chralt98]

The fuzz extrinsics tests need further modifications to not fail at the first error possibility 100% of the time, but for now the valid pool creation works, the code is separated and the fuzz tests should be valid for a review. Later the Shares::free_balance checks need to get the Shares::deposit (balances of an asset) in the fuzz tests.

[maltekliemann]

At least once the fuzz tests panicked with a duplicate asset. Nevermind, I ran an old build. 😬

There are a couple of low-priority critiques, mostly lack of randomization. But other than that, looks like a very useful abstraction of the workflow. 👍

[maltekliemann]

Yeah, looks good to me! 👍