selftests/ftrace: Add test to exercize function tracer across cpu hotplug

Add a test to exercize cpu hotplug with the function tracer active to
ensure that sensitive functions in idle path are excluded from being
traced. This helps catch issues such as the one fixed by commit
4b3338aa ("powerpc/ftrace: Fix stack teardown in ftrace_no_trace").

Signed-off-by: Naveen N Rao <naveen@kernel.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>