Use util.conditional_rm(filepath) rather than two lines to check the
existence of the file before deleting it.

Comment Checkable.check() to understand it better.
Add DBI tests for opening a database on various invalid file types --
socket, fifo, text file, etc.

Check for symlink before directory in util.conditional_rm().
Set default database name in one place.

Move the test set of Checkable.dbname out of ex_nihilo() and
get_list() into the test setup routine.

Renames: s/filename/dbname/, s/testfile/testdb/