selftests: ftrace: Add synthetic event syntax testcase

Add a testcase to check the syntax and field types for
synthetic_events interface.

Link: http://lkml.kernel.org/r/153986838264.18251.16627517536956299922.stgit@devbox



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