RocksDBStateBackendTest doesn't cover the incremental checkpoint code path

In RocksDBStateBackendTest there's a parameterized variable enableIncrementalCheckpointing, but after we refactor the backend build into the builder pattern in FLINK-10043 we didn't respect this variable. To fix this, we should add a setEnableIncrementalCheckpointing(enableIncrementalCheckpointing) invocation before building the backend in RocksDBStateBackendTest.setupRocksKeyedStateBackend.