tests/sizes: Improve metadata validation
tests/sizes: Improve metadata validation

Ensure all 3 of the checksum, compressed size and uncompressed size are
correct. For repeatable objects, skip xattrs and use canonical
permissions for the commit. For the sizes, read a varint rather than
assuming they will be a single byte. To work around bugs in gjs with
byte array unpacking, manually build the array byte by byte. Split out
some helper functions to use in subsequent tests.

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree