selftests/ftrace: Save kprobe_events to test log

It may lead to kernel panic when execute the following testcase on mips:

  # cd tools/testing/selftests/ftrace
  # ./ftracetest test.d/kprobe/multiple_kprobes.tc

A preliminary analysis shows that the issue is related with

  echo 1 > events/kprobes/enable

after add the 256 probe points.

In order to find the root cause, I want to verify which probe point has
problem, so it is necessary to save kprobe_events to test log.

With this patch, we can get the 256 probe points in the test log through
the following command:

  # ./ftracetest test.d/kprobe/multiple_kprobes.tc -vvv -k

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>