pmpkg: set default mode for files in sync packages

TarInfo objects default to mode 0644 while mkfile in util.py uses 0755,
causing pacman warnings about differing permissions on tests involving
package updates.  Set the mode on TarInfo 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>