Use system dictionary for db2 tests again

The built-in word list is not long enough for all of the libdb2 tests
to run properly.  Revert d21a86e47a7cda29225013e08d060095b94b2ee7 and
go back to using the system dictionary if we find one.  However, omit
any lines from the chosen word list which contain non-alphabetical
characters.

ticket: 7860