Refactor test_api to use pytest regression plugin

[rowanhemsi]

Is your feature request related to a problem? Please describe.
As a developer, when I add new functionality or change the default behaviour, it is awkward to update the API test

Describe the solution you'd like
Implement the pytest regression test framework for the end-to-end regression tests

Describe alternatives you've considered
Current functionality works but is more clunky and doesn't follow standard practice

Additional context
There may be more than one regression testing framework so check which to use