feat: Add checks for overwrite protection before running expensive operations

[SoloSynth1]

This PR adds a file overwrite check in expensive operation i.e. evaluate before the expensive calls (e.g. calls to APIs) happens.

Now, if the command runs when supplied with a -e/--export-report-to argument AND without a -o/--overwrite argument, it will check if a file has existed in the path. If yes, it will raise error and exit WITHOUT running the API calls.

This PR also adds a relevant test on the check, and re-organized all test fixtures into a root-level conftest.py.

Closes #163.

[JohnShiuMK]

@SoloSynth1 not sure if we can avoid these warnings, other than that, looks good to me