SAT: compute unit test coverage

[alafanechere]

Tell us about the problem you're trying to solve

(Not to be confused with https://github.com/airbytehq/airbyte-internal-issues/issues/326)
When iterating on SAT I want to measure the coverage of unit tests I wrote to test the behavior of new SAT tests.

Describe the solution you’d like

• Install pytest-cov on source-acceptance-test
• Add the --cov options by default to get coverage when running python -m pytest unit_tests
• Get a coverage report on each test run (ignore fully covered or empty files from the report)
• Show a failure if the coverage decreases.