Stop truncating ex-MyISAM tables in tests

We were doing this on MySQL because the hits tables were using MyISAM,
which doesn't support transactions (which are the default strategy for
`database_cleaner`). There shouldn't be any reason to do this on PostgreSQL,
so let's stop.

For some tests which have a lot of setup, we have been using `testing_before_all`
to indicate that their test data should be cleaned up only once at
the end, and with truncation, to speed up test runs. This commit assumes that:

- that's still a good thing to do for those cases;
- we have always been consistent about using `truncate_everything`
  for only MyISAM reasons, and `testing_before_all` for tests with lots
  of data.