fixed faiss integ tests (#5808)
Fixes # 5807
Realigned tests with implementation.
Also reinforced folder unicity for the test_faiss_local_save_load test
using date-time suffix
#### Before submitting
- Integration test updated
- formatting and linting ok (locally)
#### Who can review?
Tag maintainers/contributors who might be interested:
@hwchase17 - project lead
VectorStores / Retrievers / Memory
-@dev2049
