Better test suite fix for config changes on test models

### WHAT

Introduce a better fix for resetting test model configs in the test suite:

  * `setup`: remember the original test model configs
  * `teardown`: ensure we reset the test model configs to their originals, after each test has run

### WHY

Failures surfaced in Rails 4.1 because some tests would change the test model configs but not reset them (until this temp fix: https://github.com/binarylogic/authlogic/pull/435). This is a better fix, which wraps all test methods with a remember/reset strategy.