Turn off PV for compactblocks_2.py (#2051)
Turn off PV for compactblocks_2.py (#2051)

Occassionaly in a debug build the chaintip will be slow to update
causing build_block_on_tip() to build a block on the previous
chaintip and causing the test to fail. Turning PV off keeps the
messages processing in sequential order so we don't have a header
getting processed before the previous block has finished procesing.
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