Summary
Remove checks for the existence (or non-existence) of specific files in the storage_source cache directory from the WiredTiger-level tiered storage tests in the python suite.

If there are important test cases that are only covered in those tests, those cases should be moved to the storage_source tests, such as test_tiered06.py or test_s3_connection.cpp

Motivation

The choice of whether and how to cache content from object storage on the local file system is implemented in the storage_source. So tests at the WiredTiger API level shouldn't embed assumptions about what is or isn't cached and how it is cached.  

If we feel it should be a requirement for all storage sources to implement a cache in this way, then we should implement that functionality above the storage source.