selftests/ftrace: Add tracing/error_log testcase

Add a testcase verifying basic tracing/error_log functionality.

Link: http://lkml.kernel.org/r/bf1c0d47a24672df945331462682d96296d1ab28.1554072478.git.tom.zanussi@linux.intel.com

Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>