Consolidate tests in tox.ini and other testing enhancements.

 * pyrseas/testutils.py (remove_temp_files): New function to be used
   by test cases.  (DatabaseToMapTestCase.remove_tempfiles): New
   method.  (DbMigrateTestCase.to_map): Use TEST_DIR instead of
   tempfile.gettempdir().  (DbMigrateTestCase.remove_tempfiles):
   Refactor code out to remove_temp_files.
 * tests/dbobject/test_extern_file.py
   (ExternalFilenameMapTestCase.setUp): Remove temporary files before
   running tests.
 * tests/functional/pagila-schema.sql: Remove COMMENT on public schema
   (caused problems with other tests).
 * tox.ini: Run all tests (dbobject, functional and relation).