Test corpus update, possibly use stricter checking for file changes.

[cproctor]

Several reported issues whose cause we never resolved appear related to changing corpus files while editing. We need:

1. To confirm that the expected behavior of corpus update is exercised in the tests.
2. To have a clearer story for when and how corpus files are allowed to change.
3. To enforce that policy. Perhaps we should be checking the file hash before and/or after edits.