[12.x] Add tests for PhpRedisClusterConnection flushdb method

[miladev95]

This PR adds test coverage for PhpRedisClusterConnection methods.

Added tests:

• testItFlushesAllMasterNodes - verifies sync flush calls flushdb on all master nodes
• testItFlushesAllMasterNodesAsync - verifies async flush calls rawCommand with flushdb async on all master nodes
• testItReturnsFalseWhenCursorIsZeroAndResultIsEmpty - verifies scan returns false when cursor is 0 and Redis returns no results