If we fail to open a tablespace while looking for FILE_CHECKPOINT, we
set the corruption flag. Specifically, if encryption key is missing, we
would not be able to open an encrypted tablespace and the flag could be
set. We miss checking for this flag and report "Missing FILE_CHECKPOINT"

Address review comment to improve the test. Flush pages before starting
no-checkpoint block. It should improve the number of cases where the
test is skipped because some intermediate checkpoint is triggered.