[test] do not parse ls output for file size; NFCI

Parsing `ls -l` output to obtain the size of a file is unreliable; the
exact output format is not specified, and some user or group names may
contain multiple words, causing `cut -f5 -d' '` to extract an incorrect
value. `wc -c`, on the other hand, is portable, and there are precendents
of its use in test cases.