[EuiCodeBlock] Improve the Jest testing suite

[weronikaolejniczak]

The purpose of this task is to improve the testing suite of the EuiCodeBlock component.

---

For context: #8176 (comment)

Reevaluate the Jest tests written for the component. E.g. instead of asserting everything with a snapshot, it might bring more value to use a matcher against the component state and behavior.

packages/eui/src/components/code/code_block.test.tsx