[go-migration] Refactor common context, generate mocks, fix hollow unit tests for supply/finalize

[kiril-keranov]

The PR introduces the following changes:

• Refactor common context struct to use newly introduced interfaces to libbuildpack functionalities in order to be able to generate mocks. The mocks are further used in supply unit tests.
• Fix the supply phase unit tests that always passed and tested nothing. The generated mocks provide the ability to run supply phase without doing real download of dependencies.
  • Fixed supply test case for Tomcat container to test supply.Run(..)
  • Fixed supply test case for Spring Boot container to test supply.Run(..)
  • Fixed supply test case for Groovy container to test supply.Run(..)
• Fix the finalize phase unit tests that always passed and tested nothing.
  • Fixed finalize test case for Tomcat container to test finalize.Run(..)
  • Fixed finalize test case for Spring Boot container to test finalize.Run(..)
  • Added finalize test case for Groovy container testing finalize.Run(..)

Addressing this issue

Additional change is also adjusting the cf-metrics-exporter tests