Add option to skip reseeding fixture data.

For those who use transactional fixtures, there's actually no real need to truncate the test database and reseed the fixtures every time the test suite runs, because the test data should only change when the fixtures are updated.

This additional `reseed_db` option allows people to turn off Rails automatic truncation and reseeding of test data when the fixtures are loaded, speeding up the time it takes to run tests, without losing any of the functionality of fixtures.

This, paired with [`rake test:all`](https://github.com/rails/rails/blob/master/railties/lib/rails/test_unit/testing.rake#L18-L22) (which contrary to `rake test` doesn't trigger `rake test:prepare` which purges the test DB) will ensure fixture data is persisted between test runs.