test/encoding: run readable.sh with a sliding window of jobs

The old scheduler spawned nproc jobs, then waited for the whole batch to
finish before starting the next, so cores that finished early sat idle
waiting on the slowest type in the batch. Keep a sliding window instead:
once the window is full, reap a single finished job before spawning the
next, which keeps the cores busy when per-type runtimes vary widely.

While here, pass vdir/arversion into test_object explicitly instead of
reading the loop globals, and give each job its own result file instead
of reusing slot-numbered files that the sliding window would otherwise
hand out to overlapping jobs.

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>