[LTP] [PATCH 1/1] tst_test.sh: Fix check for $TST_DEVICE

[Cyril Hrubis]

Hi!
> When tst_device acquire call fails, it outputs into stdout,
> therefore the content is actually help message.
> => Check for file presence rather than variable not being empty.
> 
> Fixes: c5ac5d712 tst_test.sh: Make use of tst_device helper

Good catch, I guess that it also wouldn't harm if we changed the helper
to print the help message to stderr as well.

-- 
Cyril Hrubis
chrubis@suse.cz