Fixed model export logging + moved test data directory

[alex-dinh]

This PR fixes the logging during model exporting jobs (paddle2onnx/mapper/exporter.cc), particularly for when a model is not supported yet. There were many lines of code that should have logged important information in case of model conversion failure, but actually printed nothing. This should help with debugging and expanding support for new models in the future.

Output for an unsupported model before this change:

I0115 21:06:27.243992 77515 program_interpreter.cc:243] New Executor is Running.
[Paddle2ONNX] Start parsing the Paddle model file...
[ERROR][Paddle2ONNX] Due to the unsupported operators, the conversion is aborted.

Example output for an unsupported model after this change:

I0115 21:04:24.957676 76988 program_interpreter.cc:243] New Executor is Running.
[Paddle2ONNX] Start parsing the Paddle model file...
[Paddle2ONNX] Unsupported tensor operators were found:
- broadcast_tensors
[ERROR][Paddle2ONNX] Due to the unsupported operators, the conversion is aborted.

Additionally, this PR also changes the test data directory. Prior to this change, running tests locally creates many folders and files in the root directory, resulting in a lot of clutter. Instead, tests now create temp files under <project root>/test_data/

[paddle-bot]

Thanks for your contribution!