Database tasks can skip test database using SKIP_TEST_DATABASE

In Rails 6.1, Rails will skip modifications to the test database if SKIP_TEST_DATABASE is set to true.

Without the environment variable

> bundle exec rake db:create
Created database 'app_name_development'
Created database 'app_name_test'

With the environment variable

> SKIP_TEST_DATABASE=true bundle exec rake db:create
Created database 'app_name_development'

As we can see in the first example, both a development and a test database were created, which is unexpected when directly invoking db:create. One obvious solution to this problem is to force the development environment to only create a development database. However this solution will break bin/setup as mentioned in this commit. Hence the need for an environment variable to skip test database creation.

Check out the pull request for more details.