onnxruntime

Enable C# test load models with more complex directories. (#13251)

### Description
Currently, C# test only load models with the directory structure as
`{modelroot}->{opsetXX}->{modelname}->{.onnx}`
In this PR, C# test can load models from
`{modelroot}->{model-source}->{opsetXX}->{modelname}->{.onnx}`

### Motivation and Context
There're multiple sources of testing models.
1. model zoo (Not in official image)
2. 1st party models
3.  models with contrib-ops
4.  others.

It'd better to insert a mid-directory for new sources.

**This PR is compatible with current models.**
From
https://dev.azure.com/onnxruntime/onnxruntime/_build/results?buildId=776643&view=logs&j=6df8fe70-7b8f-505a-8ef0-8bf93da2bac7&t=e7d9f128-b630-5ee6-a99e-2fca70d04619&l=79
the test result is same as master build `Passed: 583, Skipped: 14,
Total: 597`

**model zoo models (mounted in ..\models\zoo) could be loaded**
And from this test workflow, it can load both existing models and models
from model zoo.

https://dev.azure.com/onnxruntime/onnxruntime/_build/results?buildId=777018&view=logs&j=6df8fe70-7b8f-505a-8ef0-8bf93da2bac7&t=e7d9f128-b630-5ee6-a99e-2fca70d04619
Skipping failed models will be in other PRs