Needed for #1617 
Related to form-data/form-data#558

Using the native Node 18+ FormData (using it in tests).
It turned out that in version 18.0.0 (only), `FormData::set` and
`::append()` do not take the MIME type of the `Blob` into account, and
it's an empty string, which always lead to `application/octet-stream`
instead of actual MIME type.

`Blob::type` is readonly via getter-only approach, so there is no way to
hack it.
So I disabled the MIME type check for Node 18.0.0 test.