selftests/ftrace: Fix multiple kprobe testcase

Fix multiple kprobe event testcase to work it correctly.
There are 2 bugfixes.
 - Since `wc -l FILE` returns not only line number but also
   FILE filename, following "if" statement always failed.
   Fix this bug by replacing it with 'cat FILE | wc -l'
 - Since "while do-done loop" block with pipeline becomes a
   subshell, $N local variable is not update outside of
   the loop.
   Fix this bug by using actual target number (256) instead
   of $N.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>