feat(bdk-sqlx): Add more tests

[f3r10]

will close #5

[ValuedMammal]

Thanks for porting the tests over. It will be easier to merge if you drop the merge commit 52298b2 and rebase the changes onto the master branch instead. For now these are my high level thoughts

• @matthiasdebernardini raised a good point recently: as a database implementation we don't intend on storing private keys so it's probably better to use tpub descriptors for the tests and examples
• For the two-wallet test it would be good to check that wallets persisted with different data do not in fact return the same data back (we'll probably need the changes from #10 to make this work)
• I think syncing with electrum would be better suited as a runnable example than a unit test. A good way to test the library would be to construct a bdk_wallet::ChangeSet with some arbitrary data, then write it to the database and check the state of the wallet after loading.

[ValuedMammal]

ACK 1170bdd