ovsdb: Check that ovsdb-server truncates corrupted database logs.

When ovsdb-server reads a database that is corrupted at the log level
(that is, when ovsdb_log detects the corruption by checking the SHA-1 hash
of the record or JSON parser error reporting), then writing to the database
should discard the corrupted data and thereby fix the problem for future
ovsdb-server runs.

This already worked OK.  This just adds an extra test.