Setup unittests for restx refactoring

[wl2484]

Goal

Reorganize test cases for flask-restx refactoring. After adopting this change, test case fixes required for flask-restx refactoring would generally be changing the base_url value.

Changes

1. Add BaseTestCase class with class variable base_url
2. Remove duplicate tests
3. Add create shopcart happy path test case for the sake of test coverage
4. Retire global variable NONEXIST_SHOPCART_ID in test_routes.py
5. Declare and apply DEFAULT_CONTENT_TYPE in tests

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (69439b8) 98.98% compared to head (28590b1) 98.98%.

:exclamation: Current head 28590b1 differs from pull request most recent head c8315f1. Consider uploading reports for the commit c8315f1 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## sc84_swagger_item_model #139 +/- ## ======================================================== Coverage 98.98% 98.98% ======================================================== Files 8 8 Lines 393 393 ======================================================== Hits 389 389 Misses 4 4 ```

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