Convert zsk rollover test case to pytest

Convert zsk rollover test case to pytest

Move the 'zsk-prepub' zones to the rollover test dir and convert ZSK
rollover test to pytest.

We need a way to signal a smooth rollover is going on. Signatures are
being replaced gradually during a ZSK rollover, so the existing
signatures of the predecessor ZSK are still being used. Add a smooth
operator to set the right expectations on what signatures are being
used.

Setting expected key relationships is a bit crude: a list of two
elements where the first element is the index of the expected keys that
is the predecessor, and the second element is the index of the expected
keys that is the successor.

We are changing the KSK lifetime to unlimited in this test case as it
is of no importance.