Add a convenience macro invoke subroutines

To properly fail a test in a subroutine, return value needs to be passed up.
This snippet makes it short.

PUBLISHED_FROM=2f6328b5eb2921a4caa53d8887c02f73a5d817ea

src/common/test_util.h

@@ -165,6 +165,12 @@ void _strfail(const char *a, const char *e, int len);
     }                                                                \
   } while (0)
 
+#define CHECK_CALL(call)     \
+  do {                       \
+    const char *r = (call);  \
+    if (r != NULL) return r; \
+  } while (0)
+
 #ifndef MG_ENABLE_POLL_UNTIL
 #define MG_ENABLE_POLL_UNTIL 0
 #endif