selftests: ftrace: Add a testcase for string type with kprobe_event

Add a testcase for string type with kprobe event.
This tests good/bad syntax combinations and also
the traced data is correct in several way.

Link: http://lkml.kernel.org/r/152129038381.31874.9201387794548737554.stgit@devbox

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