Replacing form-data with native FormData of Node 18

[RobinTail]

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.

[coveralls-official]

coverage: 100.0%. remained the same
when pulling 56941ca on no-form-data
into aadf5ad on master.