Test backends separately

[shioyama]

Currently, we test backends together in a test run. This is problematic for cases where, for example, a required file is not loaded within a class, since this can be missed when all backends are run together (in case that another backend loads it).

To avoid this, each test run should run against only one backend. This will result in many many more test runs, so this could be done just for e.g. the latest version of Ruby, and only on postgres, to minimize the number of extra runs.