selftests: ftrace: Output only to console with "--logdir -"

Output logs only to console if "-" is given to --logdir
option. In this case, ftracetest doesn't record any log
on the disk, and all logs immediately shown (including
all command logs.) Since there is no "tee" in the middle
of command and console, it outputs the log really soon.

This option is useful only when the console is logged.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Steven Rostedt (VMware) <srostedt@goodmis.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>