selftests: ftrace: Add probe event argument syntax testcase

Add a testcase for probe event argument syntax which
ensures the kprobe_events interface correctly parses
given event arguments.

Link: http://lkml.kernel.org/r/152129033679.31874.12705519603869152799.stgit@devbox

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