Tests: Update QONNX export tests to include property, accuracy validation

[nickfraser]

Current QONNX tests only test that export run without error. It would be much better to include some tests to check that the exported node properties match what we expect, and that the QONNX runner produces the correct output, accurate to some delta. Suggested in #986.

Could also consolidate the testing effort in finn. FYI @Giuseppe5, @auphelia.