Consider image styles badge generation tests

Before it failed like:

    $ ruby -Ilib:test
    test/inch_ci/worker/project/build/generate_badge_test.rb
    Run options: --seed 12918

    # Running:

    F

    Finished in 0.293940s, 3.4021 runs/s, 3.4021 assertions/s.

      1) Failure:
    InchCI::Worker::Project::Build::GenerateBadge#test_0001_should only
    create code objects when they change
    [test/inch_ci/worker/project/build/generate_badge_test.rb:15]:
    Failed assertion, no message given.

    1 runs, 1 assertions, 1 failures, 0 errors, 0 skips

After this commit is passes:

    $ ruby -Ilib:test
    test/inch_ci/worker/project/build/generate_badge_test.rb
    Run options: --seed 21755

    # Running:

    .

    Finished in 0.323241s, 3.0937 runs/s, 12.3747 assertions/s.

    1 runs, 4 assertions, 0 failures, 0 errors, 0 skips