selftests: ftrace: Add -l/--logdir option

In my virtual machine setup, running ftracetest failed on creating
LOG_DIR on a read-only filesystem.  It'd be convenient to provide an
option to specify a different directory as log directory.

Link: http://lkml.kernel.org/r/20170417024430.21194-4-namhyung@kernel.org

Cc: Ingo Molnar <mingo@kernel.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>