[js/web] support external data in npm test (#19377)
### Description
support external data in npm test.
This allows test runner to detect whether an external data is available
in the test folder, and if it is, load it as external data
automatically.
this feature does not parse every model to figure out whether the model
has external data. the following comments in code explained how to
determine whether should parse the model file.
```js
// for performance consideration, we do not parse every model. when we think it's likely to have external
// data, we will parse it. We think it's "likely" when one of the following conditions is met:
// 1. any file in the same folder has the similar file name as the model file
// (e.g., model file is "model_abc.onnx", and there is a file "model_abc.pb" or "model_abc.onnx.data")
// 2. the file size is larger than 1GB
```
