Make getLatestSnapshot and removeState member functions in TestHarness's Node.py #43
Description
While working on writing a snapshot test for Savanna transition, I noticed getLatestSnapshot and removeState were implemented in nodeos_irreversible_mode_test.py, nodeos_snapshot_diff_test.py, and ship_streamer_test.py. One of the implementation missed the snapshot schedule config check.
We should move those two functions to Node class so that they can be reused by different tests.