ktest: add 2nd parameter of run_command() to set the redirect target file

ktest: add 2nd parameter of run_command() to set the redirect target file

If we'd like to set the redirect target file of run_command(),
we should define $redirect before calling this function and should undef it
after calling this function. Since it's user-unfriendly, add 2nd parameter of
run_command() for this purpose.

Link: http://lkml.kernel.org/r/87vbvwokq8.wl%satoru.takeuchi@gmail.com
Signed-off-by: Satoru Takeuchi <satoru.takeuchi@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>