bpo-45500: Rewrite test_dbm (GH-29002)

bpo-45500: Rewrite test_dbm (GH-29002)

* Generate test classes at import time. It allows to filter them when
  run with unittest. E.g: "./python -m unittest test.test_dbm.TestCase_gnu -v".
* Create a database class in a new directory which will be removed after
  test. It guarantees that all created files and directories be removed
  and will not conflict with other dbm tests.
* Restore dbm._defaultmod after tests. Previously it was set to the last
  dbm module (dbm.dumb) which affected other tests.
* Enable the whichdb test for dbm.dumb.
* Move test_keys to the correct test class. It does not test whichdb().
* Remove some outdated code and comments.