Truncate tables when they already exist.

This solves an annoying workflow interuption when the following happens:

1. Create a test case and make mistake that creates a fatal error.
2. Fix the error and re-run the test.

If your fixtures use uuid keys, you'll get an annoying duplicate primary
key error that should really not happen.
Merge pull request #4622 from cakephp/fixture-clear

Truncate tables when they already exist.