There are two ways to run unit tests. `bundle exec rake test` and
`bin/test`. Active Record `bundle exec rake test` only executes its
own adapter specific tests. on the other hand, `bin/test` executes all
adapter specfic tests then some test files need to have `if
current_adapter?`, which can be removed.

- Removed `require_relative "../../tools/test"` to require its own one
- `default_test_exclude_glob` excludes all of adapter specific tests.
- `list_tests(argv)` adds its own adapter specific tests
- `adapter_short` method extracted to get the short hand adapter name
  like `sqlite3`, `mysql2` and `postgresql` via `ENV["ARCONN"]`