Move WASM_ENABLE_SPEC_TEST to correct place (#1447)

Fix compilation warning:
```
wamr/product-mini/platforms/nuttx/../posix/main.c:622:5: warning: "WASM_ENABLE_SPEC_TEST" is not defined, evaluates to 0 [-Wundef]
  622 | #if WASM_ENABLE_SPEC_TEST != 0
```

Signed-off-by: Huang Qi <huangqi3@xiaomi.com>

core/config.h

@@ -282,6 +282,10 @@
 #define WASM_ENABLE_GLOBAL_HEAP_POOL 0
 #endif
 
+#ifndef WASM_ENABLE_SPEC_TEST
+#define WASM_ENABLE_SPEC_TEST 0
+#endif
+
 /* Global heap pool size in bytes */
 #ifndef WASM_GLOBAL_HEAP_SIZE
 #if WASM_ENABLE_SPEC_TEST != 0
@@ -383,10 +387,6 @@
 #endif
 #define BLOCK_ADDR_CONFLICT_SIZE 2
 
-#ifndef WASM_ENABLE_SPEC_TEST
-#define WASM_ENABLE_SPEC_TEST 0
-#endif
-
 /* Default max thread num per cluster. Can be overwrite by
     wasm_runtime_set_max_thread_num */
 #define CLUSTER_MAX_THREAD_NUM 4