tests: fix --unbuffered mode with ASAN

tests: fix --unbuffered mode with ASAN

Unfortunately, ASAN is pretty sensitive to LD_PRELOAD, but stdbuf from
coreutils is based on LD_PRELOAD. So, I have replaced stdbuf with
unbuffer (from expect pkg).

The another problem is "${args[@]}" "$@" which does not work as expected.

Well, this patch sucks. It would be nice to have things in the way
how it has been original expected by Patrick's patch, but ...

Signed-off-by: Karel Zak <kzak@redhat.com>