Fix test failures where assumed data does not exist

When adding test coverage for another branch, I ran into failures with the test_foreign_key and test_datetime_miliseconds test methods.

I've updated each of these methods to improve the likelihood that the data required by each assertion exists.  An org may not always have a CronTrigger that has a PreviousFireTime set, or an org may have a contact owned by a user other than the user used by django-salesforce.