selftests: Remove forced unbuffering for test running

As it turns out, the "stdbuf" command will actually force all
subprocesses into unbuffered output, and some implementations of "echo"
turn into single-character writes, which utterly wrecks writes to /sys
and /proc files.

Instead, drop the "stdbuf" usage, and for any tests that want explicit
flushing between newlines, they'll have to add "fflush(stdout);" as
needed.

Reported-by: Takashi Iwai <tiwai@suse.de>
Fixes: 5c069b6d ("selftests: Move test output to diagnostic lines")
Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>