[Autotuner] Add checkpoint save/load/resume tests

Add test_autotuner_checkpoint.py with 17 tests covering checkpoint
save/load cycles for all search algorithms (PatternSearch,
LFBOPatternSearch, LFBOTreeSearch, DE, DESurrogateHybrid).

stack-info: PR: #1920, branch: yf225/stack/90

Author: yf225