selftests/ftrace: Add --console hidden option

Add --console hidden option for debug test cases.

This option allows to put "sh" or something else
when the test case hits a bug.

For example, if you find a testcase which doesn't
pass, you can insert sh for interactive debug as below

  -----
  #!/bin/sh
  # description: sample test case

  good-command
  suspicious-wrong-command
  sh # <- add this for interactive debug
  -----

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>