Refactor series (de)serialization tests

I've also removed some redundant tests and changed others that were
not actually testing what they should, so as a consequence we went
from 76.6% coverage to 77%.

I think there's still room for improvement in those tests, but the
diff was already quite big so I decided to stop here and save the
other changes for another branch

Before:
github.com/monetas/btcwallet/waddrmgr/db.go:	deserializeSeriesRow		89.7%
github.com/monetas/btcwallet/waddrmgr/db.go:	serializeSeriesRow		93.1%

After:
github.com/monetas/btcwallet/waddrmgr/db.go:	deserializeSeriesRow		94.9%
github.com/monetas/btcwallet/waddrmgr/db.go:	serializeSeriesRow		100.0%

Can you make it a bit more clear what we are testing here? Maybe serialized data contains less bytes than the smallest possible series.