selftests/ftrace: Use loopback address instead of localhost

Use raw loopback address instead of localhost, because
"localhost" can depend on nsswitch and in some case
we can not resolve the localhost.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>

tools/testing/selftests/ftrace/test.d/functions

@@ -103,3 +103,9 @@ initialize_ftrace() { # Reset ftrace to initial-state
     clear_trace
     enable_tracing
 }
+
+LOCALHOST=127.0.0.1
+
+yield() {
+    ping $LOCALHOST -c 1 || sleep .001 || usleep 1 || sleep 1
+}