Improve code coverage for scan, especially for N48

[thompsonbry]

N48 tends to be difficult to hit.

See

unodb/test/test_art.cpp

Lines 304 to 354 in f896f77

	TYPED_TEST(ARTCorrectnessTest, Node48) {
	unodb::test::tree_verifier<TypeParam> verifier;
	verifier.insert_key_range(0, 17);
	verifier.check_present_values();
	verifier.check_absent_keys({17});
	#ifdef UNODB_DETAIL_WITH_STATS
	verifier.assert_node_counts({17, 0, 0, 1, 0});
	verifier.assert_growing_inodes({1, 1, 1, 0});
	#endif // UNODB_DETAIL_WITH_STATS
	}
	TYPED_TEST(ARTCorrectnessTest, FullNode48) {
	unodb::test::tree_verifier<TypeParam> verifier;
	verifier.insert_key_range(0, 48);
	verifier.check_present_values();
	verifier.check_absent_keys({49});
	#ifdef UNODB_DETAIL_WITH_STATS
	verifier.assert_node_counts({48, 0, 0, 1, 0});
	verifier.assert_growing_inodes({1, 1, 1, 0});
	#endif // UNODB_DETAIL_WITH_STATS
	}
	TYPED_TEST(ARTCorrectnessTest, Node48KeyPrefixSplit) {
	unodb::test::tree_verifier<TypeParam> verifier;
	verifier.insert_key_range(10, 17);
	#ifdef UNODB_DETAIL_WITH_STATS
	verifier.assert_node_counts({17, 0, 0, 1, 0});
	verifier.assert_growing_inodes({1, 1, 1, 0});
	verifier.assert_key_prefix_splits(0);
	#endif // UNODB_DETAIL_WITH_STATS
	// Insert a value that does share full prefix with the current Node48
	verifier.insert(0x100020, unodb::test::test_values[0]);
	verifier.check_present_values();
	verifier.check_absent_keys({9, 27, 0x100019, 0x100100, 0x110000});
	#ifdef UNODB_DETAIL_WITH_STATS
	verifier.assert_node_counts({18, 1, 0, 1, 0});
	verifier.assert_growing_inodes({2, 1, 1, 0});
	verifier.assert_key_prefix_splits(1);
	#endif // UNODB_DETAIL_WITH_STATS
	}

and add iterator / scan operations into these tests to improve the code coverage in a focused manner.