[QNN EP]Add UT for cached Qnn context binary (#16184)
### Description
1. Add UT for cached Qnn context binary
2. Minor change: set model path to "" if model_path is not available
since the model could be loaded from buffer instead of Onnx file
### Motivation and Context
support more scenario
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Co-authored-by: Adrian Lizarraga <adlizarraga@microsoft.com>
