Some migration tests used structs and methods that are expected
to change and so will not be backward compatible.

This change replaces those with structs and methods specific to
the migration tests so that the tests will be unaffected by DB
schema changes. At the same time, these new structs and methods
simplify some of the setup by allowing to store new coins,
including their confirmation and spend status, in a single
DB operation.

I found that making changes to the transactions used in the test
can affect the order in which coins are returned, probably due
to the txid changing.

The new columns in the transactions table will be populated for
new transactions by the poller, while existing rows will be
updated in a subsequent migration.

New and existing coins will all have `is_from_self` set to false
due to the default column value.

None of these columns will be used by the wallet at this stage.

This populates the new columns from the previous migration
for existing rows and then maintains them in the poller moving
forward.