Remove onnx/backend/test from package

[hmaarrfk]

Comment:

Man looking through the logs, the onnx/backend/test directory killed me.

It just copies 1000's of items, that are likely not that useful to the typical end user.

Would it be appropriate to remove them from the package.

Screenshot taken from v1.15.0

[cbourjau]

Corresponding upstream issue: onnx/onnx#5925

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