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.

FWIW:

with self.reseed_db = true (default) :
```
mbp4:app mike.campbell$ time bin/rake test
Run options: --seed 34380
Started

Finished in 53.98907s
16 tests, 74 assertions, 0 failures, 0 errors, 0 skips

real	1m30.791s
user	0m0.527s
sys	0m0.304s
```

with self.reseed_db = false :
```
mbp4:app mike.campbell$ time bin/rake test:all
Run options: --seed 43004
Started

Finished in 43.51898s
16 tests, 74 assertions, 0 failures, 0 errors, 0 skips

real	0m56.395s
user	0m0.557s
sys	0m0.365s
```

That's an third knocked off the time of my capybara acceptance test suite.

Is this change related to reseeding the database?