tests: use pdiff instead of diff for portability
"diff" command might cause redundant message, "No differences
encountered" on Solaris for example. But suppressing option like "-q"
isn't portable, because POSIX specification doesn't define it.
pdiff script was introduced by b59ef0c21405 to stabilize output of
standard diff command on each platforms.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.