Support alternate rails environment names for tests

[justindoody]

Changes

Switches to using db:purge over db:test:purge.

db:purge:

desc "Empty the database from DATABASE_URL or config/database.yml for the current RAILS_ENV (use db:purge:all to purge all databases in the config). Without RAILS_ENV it defaults to purging the development and test databases."

db:test:purge hardcodes loading configuration for an environment named test. db:purge will load the environment for whatever RAILS_ENV is set to (or fallback).

Here within parallel_tests purge_before_load is used on lines that support passing through alternate env's (RAILS_ENV=#{ParallelTests::Tasks.rails_env}).

Usecase

I'm trying to run parallel_tests in it's own rails environment i'm naming parallel_test -- this is so I can specify a few custom environment options directly for parallel runs and so i can run one of tests against a separate environment (and db config) while my parallel test suite is running.

This should be backwards compatible while expanding flexibility.

Other

Wasn't sure how you are handling versioning so did a patch level bump.

[grosser]

sounds good ... drop does about the same, so if the logic did not work people would have already complained :)

[grosser]

v2.26.2