ftracetest: Clear trace buffer after running kprobe testcases

The kprobe testcases create, use and delete dynamic events during the
test but didn't clear the trace buffer so it'll leave the result after
it finishes.

  # ./ftracetest
  ...

  # cat trace
  # tracer: nop
  #
  # entries-in-buffer/entries-written: 2/2   #P:12
  #
  #                              _-----=> irqs-off
  #                             / _----=> need-resched
  #                            | / _---=> hardirq/softirq
  #                            || / _--=> preempt-depth
  #                            ||| /     delay
  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
  #              | |       |   ||||       |         |
        ftracetest-26474 [009] d..1 79417.143782: Unknown type 1099
        ftracetest-26498 [009] d..1 79417.208034: Unknown type 1101

Link: http://lkml.kernel.org/r/1415239470-28705-2-git-send-email-namhyung@kernel.org
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>