test: fix n-api addon build warnings

Fixed an MSVC warning on Windows:
* test_general.c - Lossy conversion from int64 to double, explicitly
  casting to double resolved the warning

PR-URL: #21808
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/addons-napi/test_general/test_general.c

@@ -216,7 +216,7 @@ static napi_value testAdjustExternalMemory(napi_env env, napi_callback_info info
   int64_t adjustedValue;
 
   NAPI_CALL(env, napi_adjust_external_memory(env, 1, &adjustedValue));
-  NAPI_CALL(env, napi_create_double(env, adjustedValue, &result));
+  NAPI_CALL(env, napi_create_double(env, (double)adjustedValue, &result));
 
   return result;
 }