Module api tests for RM_Call

Adding a test for coverage for RM_Call in a new "misc" unit
to be used for various short simple tests

also solves compilation warnings in redismodule.h and fork.c

tests/modules/fork.c

@@ -1,6 +1,10 @@
 #define REDISMODULE_EXPERIMENTAL_API
-#include "redismodule.h"
 
+/* define macros for having usleep */
+#define _BSD_SOURCE
+#define _DEFAULT_SOURCE
+
+#include "redismodule.h"
 #include <string.h>
 #include <assert.h>
 #include <unistd.h>