Add test to check `WalletPersister` implementation

[110CodingP]

My initial thought that bdk-cli is enough for this is not correct. Good to have a simple test inside bdk_redb than having to modify bdk-cli every time we change the implementation, until we have the general persistence test suite!

[110CodingP]

Perhaps the tests in #16 are unnecessarily complicated for just checking the persister impl, maybe #19 is better ?

[110CodingP]

#19 should be enough to fix this. #20 will also be needed.