Tests should fail when snapshots are added in CI environment

[deanohyeah]

Currently, if a snapshot doesn't exist it is automatically added and passes.
https://github.com/americanexpress/jest-image-snapshot/blob/main/src/diff-snapshot.js#L207

To be inline with jest snapshots it should fail when CI=true and new snapshots are added.
https://jestjs.io/docs/snapshot-testing#are-snapshots-written-automatically-on-continuous-integration-ci-systems

snapshots in Jest are not automatically written when Jest is run in a CI system without explicitly passing --updateSnapshot. It is expected that all snapshots are part of the code that is run on CI and since new snapshots automatically pass, they should not pass a test run on a CI system. It is recommended to always commit all snapshots and to keep them in version control.