Faster tests by moving testCreatePolicy() and testLoadPolicy() out of base test class

PasswordPolicyTestCase was being used as a base test class with helper functions by being extended by PasswordPolicyConstraintsTestCase() and PasswordPolicyConditionsTestCase() but that leads to extra calls to testCreatePolicy() and testLoadPolicy().

By creating a PasswordPolicyTestBase() that only does setUp() and helper functions I was able to reduce the simpletest run time by 1 minute from 2 minute and 40 seconds.

Attached patch does this. I think PasswordPolicyConstraintsTestCase could be further sped up by running each constraint in a single test function, so I'll try that out next.