pmpkg: set default mode for files in sync packages

TarInfo objects default to mode 0644 while mkfile in util.py uses 0755
for directories, causing pacman warnings about differing permissions on
tests involving package updates.  Set the mode on TarInfo directory
objects to 0755 unless the test specifies a different mode.

Bug referenced in FS#30723.

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
util.py: replace file() with open()

open() is the standard way to open a file in python.

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>