Tests: always run Lucene's CheckIndex when shards are closed in tests and fail the test if corruption is detected

Today we only run 10% of the time, and the test doesn't fail when
corruption is detected.

I think it's better to always run and fail the test, so we can catch
any possible resiliency bugs in Lucene/Elasticsearch causing corruption.

For known tests that create corrupted indices, it's easy to set
MockFSDirectoryService.CHECK_INDEX_ON_CLOSE to false...

Closes #7730