[FIXED JENKINS-6446] Modified the scoring mechanism to achieve symmetry and consistency

Scoring mechanism is detailed below.

    Passing test
        added: +1
        removed: -1
    Failing test
        added: -1
        removed: +1
    Skipped test (no marks given or reduced)
        added: 0
        removed: 0

Following is the details of how all scenarios are handled.

    Passing Tests
        Passing test added = +1
            [Increases passing count = +1]
        Passing test removed = -1
            [Decreases passing count = -1]
        Passing an already failing test = +2
            [Decreases failing count = +1]
            [Increases passing count = +1]
        Passing an already skipped test = +1
            [Decreases skipped count = 0]
            [Increases passing count = +1]

    Failing tests
        Failing test added = -1
            [Increases failing count = -1]
        Failing test removed = +1
            [Decreases failing count = +1]
        Failing an already passing test = -2
            [Decreases passing count = -1]
            [Increases failing count = -1]
        Failing an already skipped test = -1
            [Decreases skipped count = 0]
            [Increases failing count = -1]

    Skipped tests
        Skipped test added = 0
            [Increases skipped count = 0]
        Skipped test removed = 0
            [Decreases skipped count = 0]
        Skipped an already passing test = -1
            [Decreases passing count = -1]
            [Increases skipped count = 0]
        Skipped an already failing test = +1
            [Decreases failing count = +1]
            [Increases skipped count = 0]

Note: Skipped tests are eventually ignored here, but intentionally adding calculators for them so that someone can give marks for skipped ones if required.