[qa] Simplify import-rescan.py

Get rid of partial functions so the test can be more easily extended to add
more variants of imports with options that affect rescanning (e.g. different
key timestamps).

Also change the second half of the test to send /to/ the imported addresses,
instead of /from/ the imported addresses. The goal of this part of the test was
to confirm that the wallet would pick up new transactions after an import
regardless of whether or not a rescan happened during the import. But because
the wallet can only do this reliably for incoming transactions and not outgoing
transactions (which require the wallet to look up transaction inputs) the test
previously was less meaningful than it should have been.

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