Fix intermittent test failure, reduce test time

In txn_clone.py, non-essential test transaction tx2 was asserted
to be unconfirmed, but would occasionally confirm depending on
network behavior. Fix by explicitly sharing with miner and
checking for 1 confirmation.

Reduce required conflict tests from 4 to 2, by moving one invocation
each of txn_clone.py and txn_doublespend.py to the extented test
section. This saves about 15 seconds per test run for me.