[1.2.X] Fixed #14415 -- Corrected the process of creating and destroying test...

[1.2.X] Fixed #14415 -- Corrected the process of creating and destroying test databases to prevent accidental deletion of the source database. Also improves the mirroring process to prevent some cross-connection effects. Thanks to Shai Berger for the help diagnosing and testing.

Backport of r14696 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14697 bcc190cf-cafb-0310-a4f2-bffc1f526a37