tests/sizes.js: Fix byte array unpacking

tests/sizes.js: Fix byte array unpacking

Recent GJS changed how byte arrays are unpacked with some assumptions
that they are likely strings. Manually use get_child_value() and
get_byte() to ensure the correct value is parsed when checking the
`ostree.sizes` metadata.

The upstream test is currently passing fine with GJS 1.56.2, but at
Endless we (unfortunately) have a downstream change that adds the object
type as an additional byte in the array. This is parsed incorrectly by
`deep_unpack()`. We can carry this patch downstream, but this change
makes the test more robust regardless.

Closes: #1884
Approved by: cgwalters