Adds docs/integration-test.md

[chernser]

Summary

• Adds docs/integration-test.md that describes testing strategies of different areas when integrating with client or JDBC driver.

Checklist

Delete items not relevant to your PR:

• Closes #
• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided to include in CHANGELOG
• For significant changes, documentation in https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials