Currently extensible.c is not covered by any tests. The proposed patch fixes this. Also it can serve as an example of using CustomScan and ExtensibleNode.
Firstly, in test_create_custom_scan_state() why hard coding css.slotOps = &TTSOpsBufferHeapTuple and not using the table_slot_callbacks() for the purpose and getting the real AM.
Next, in test_plan_custom_path, the last two arguments are never used.
There are no tests for nodeCopy, nodeEqual, nodeOut, and nodeRead routines.