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

[ Upstream commit 5fbdbed797b6d12d043a5121fdbc8d8b49d10e80 ]

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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>