pytest: add a C testing plugin to test libplugin

We mark the test as xfail() as it exposes that libplugin's PLUGIN_RESTARTABLE
was not taken into account !

tests/plugins/test_libplugin.c

@@ -0,0 +1,52 @@
+#include <ccan/array_size/array_size.h>
+#include <plugins/libplugin.h>
+
+
+const char *name_option;
+
+
+static struct command_result *json_helloworld(struct command *cmd,
+					      const char *buf,
+					      const jsmntok_t *params)
+{
+	const char *name;
+
+	if (!param(cmd, buf, params,
+		   p_opt("name", param_string, &name),
+		   NULL))
+		return command_param_failed();
+
+	if (!name)
+		name = name_option ? name_option : tal_strdup(tmpctx, "world");
+
+	return command_success_str(cmd, tal_fmt(tmpctx, "hello %s", name));
+}
+
+static void init(struct plugin *p,
+		  const char *buf UNUSED, const jsmntok_t *config UNUSED)
+{
+	plugin_log(p, LOG_DBG, "test_libplugin initialised!");
+}
+
+static const struct plugin_command commands[] = { {
+		"helloworld",
+		"utils",
+		"Say hello to the world.",
+		"Returns 'hello world' by default, 'hello {name}' if the name"
+		" option was set, and 'hello {name}' if the name parameter "
+		"was passed (takes over the option)",
+		json_helloworld,
+	}
+};
+
+int main(int argc, char *argv[])
+{
+	setup_locale();
+	plugin_main(argv, init, PLUGIN_RESTARTABLE, commands, ARRAY_SIZE(commands),
+	            NULL, 0, NULL, 0,
+		    plugin_option("name",
+				  "string",
+				  "Who to say hello to.",
+				  charp_option, &name_option),
+		    NULL);
+}