Improve testing for the API Mediation Layer both on & off platform

[jalel01]

The objective is to Increase confidence in the API ML by improving:

• build stability
• developer experience 
• reliability of codebase

[balhar-jakub]

As we got two stories under our belt around the topic I would like to outline in more detail what we aim to deliver and possibly reshuffle stories so that it represents the issues.

What are the key topics we are looking at?

• Better usage of the GH Actions
  • Parallelize packaging (#1361)
  • Run microservices in service containers (#1361)
  • Explore parallelization of the unit tests (#1416)
• Improved testing of the on-platform integration
  • Adding the smoke tests to the Marist runs (#1332)
  • Splitting the zOS code into a separate package with separate testing. (#1424 and #1425)
• Functional tests per component (Not covered anywhere)
  • Build the functional tests for each of the components
  • Move the functional tests to the component tests

I propose that we replace the Explore parallelization of the unit tests or the parallelization of tests in general with fixing the functional (acceptance) tests we have on Gateway and creating the functional tests for other services.

[balhar-jakub]

Will be managed in different issues.