ftrace: Add self-tests for multiple function trace users

Add some basic sanity tests for multiple users of the function
tracer at startup.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --cc kernel/trace/trace.h
Simple merge

diff --cc kernel/trace/trace_selftest.c
Simple merge

diff --cc kernel/trace/trace_selftest_dynamic.c
Simple merge