Test: always run CheckIndex after a test, and fail the test if it detects corruption

Simple pull request; all tests passed at least once.

The boolean I added is a bit hackity but seems to work; if anyone knows a cleaner way let me know ... (exceptions thrown here are simply caught & logged above).

Tests that create known corrupted shards already seem to set the boolean setting (index.store.mock.check_index_on_close) to false ...