qa: reduce wallet config impact on importprunedfunds.py
qa: reduce wallet config impact on importprunedfunds.py

Changes client parametrization and mining frequency inside
importprunedfunds.py to make sure that when wallet configuration
changes, the test still can succeed, by not allowing it to respend
unconfirmed outputs that could otherwise be reused in this test.

The alternative would be to have to change this test every time the
wallet defaults change, which is not the subject of this test.

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