test: Load new model version should not reload loaded existing model version(s)

Add the following tests on model version reload:

If a version is loaded and unmodified, then it should not be reloaded on the next load request.
If a version is loaded but modified, then it should be reloaded on the next load request.
If a version is not loaded and its model file is in the model directory, then it should be loaded on the next load request.
If a version is not loaded nor its model file is in the model directory, then it should be loaded on the next load request.
If a generic file is modified on the model directory, i.e. model_directory/common_dependency.py, then all loaded version(s) should be reloaded on the next load request.

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Start with the core PR.

The PR adds the test, see the Test plan on the core PR.

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triton-inference-server / server