Add unit test to check escrow get rewards calls correct pallet

[bvotteler]

We introduced a bug when refactoring some work as getting staked rewards called the incorrect palette’s withdrawRewards() function (ie. escrow rather than escrowAnnuity).

The method making the call: EscrowAPI.buildWithdrawRewardsExtrinsic()

Add a unit test that checks the call is made for the right pallet.