iotests: add script_initialize

iotests: add script_initialize

Like script_main, but doesn't require a single point of entry.
Replace all existing initialization sections with this drop-in replacement.

This brings debug support to all existing script-style iotests.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200331000014.11581-12-jsnow@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
[mreitz: Give 274 the same treatment]
Signed-off-by: Max Reitz <mreitz@redhat.com>