[draft] Introduce watchdog into ProcessGroupManager [4/N]: Add ProcessGroupManager unit tests for watchdog

[cameron-craig-etas]

eclipse-score#330

This is part 3 of a number of PRs, to move the watchdog ownership to ProcessGroupManager. Now that the watchdow is owned by ProcessGroupManager, we are adding some unit tests to ensure the watchdog is called as we would expect.

The following changes have been made:

• Mocks and stubs have been implemented where needed, in order to test ProcessGroupManager.
• Tests have been added to ensure the watchdog methods are called when expected.
• The GivenWhenThen structure is being used for these tests, as suggested in the Test Specification Guide.
• We do not plan to link these tests to detailed design at this stage.

[cameron-craig-etas]

• Waiting for Introduce watchdog into ProcessGroupManager [3/N]: Move ownership to ProcessGroupManager eclipse-score/lifecycle#355 to merge before rebasing this branch and opening it for review.