Adapt x/btcstaking unit tests to pre-approval flow

[KonradStaniec]

after: #117

Most x/btcstaking tests still assume old flow. We need to adapt them to new flow, and test both paths. This is also good ocasion to refactor x/btcstaking message handlers

[SebastianElvis]

Since we are doing this, what do you think about merging the test util under x/btcstaking with the test framework at https://github.com/babylonlabs-io/babylon/blob/544386204a860ee7159923b3d34174d0cf8c6049/testutil/helper, and use this framework for testing everything? This way we get rid of many mock tests

[KonradStaniec]

you mean get rid of https://github.com/babylonlabs-io/babylon/blob/main/x/btcstaking/keeper/keeper_test.go#L30 ? Tbh sounds great 👍