Move Checkable.py tests down into tests/CheckableTest.py.
- Minimized imports on touched files
- Created class Chdir() for use with the 'with' statement to manage
  directory visits
- Log and test data in tests for CrawlConfigTest
Moved tests for CrawlDBI.py down into tests/CrawlDBITest.py.
Allow DBI constructor to take dbtype arg in addition to a config
object to specify the database type to instantiate.

Provide a method on DBI (which means it's required on DBI<specific>
objects) to return a database cursor.

Rewrite tests in DBItstBase in terms of DBI rather than DBIsqlite.
Moved CrawlPlugin.py tests to tests/CrawlPluginTest.py.
Moved Dimension.py tests to tests/DimensionTest.py.