Add sitecustomize.py tests for Auto-Instrumentation

[jeremydvoss]

Is your feature request related to a problem?
If so, provide a concise description of the problem:
Yes. There are currently no unit tests for sitecustomize.py. Such tests could have caught the Windows bug I noticed recently.

Describe the solution you'd like
What do you want to happen instead? What is the expected behavior?
I propose creating unit tests for sitecustomize to make sure it works and continues to work for linux, windows, and mac environments.

Describe alternatives you've considered
Which alternative solutions or features have you considered?

Additional context
Add any other context about the feature request here.
Adding tests was suggested in this PR I proposed creating this ticket instead since no test suite currently exists.