Fix Rails environment when running tests with Ruby

I frequently run tests with `ruby`, not with a runner like `rake` or
`rails`.  When running the test with just `ruby` the `RAILS_ENV`
environment variable did not get set to "test", and this would cause the
tests to fail (and even mutate the development database!)

This commit adds integration tests for running tests with just `ruby`
and ensures the environment gets defaulted to "test".  I also added a
test to ensure that passing an environment to `-e` actually works (and
fixed that case too).

An interesting / annoying thing is that Minitest picks up it's plugins
by asking RubyGems for a list of files:

  https://github.com/seattlerb/minitest/blob/ca6a71ca901016db09a5ad466b4adea4b52a504a/lib/minitest.rb#L92-L100

This means that RubyGems needs to somehow know about the file before it
can return it to Minitest.  Since we are not packaging Rails as a Gem
before running the integration tests on it (duh, why would you do
that?), RubyGems doesn't know about the file, so it can't tell Minitest,
so Minitest doesn't automatically require it.  This means I had to
manually require and insert the plugin in our integration test.  I've
left comments about that in the test as well.

Ugh.

railties/lib/rails/test_unit/runner.rb

@@ -12,8 +12,11 @@ class Runner
 
       class << self
         def attach_before_load_options(opts)
+          ENV["RAILS_ENV"] = "test"
           opts.on("--warnings", "-w", "Run with Ruby warnings enabled") {}
-          opts.on("--environment", "-e", "Run tests in the ENV environment") {}
+          opts.on("-e", "--environment ENV", "Run tests in the ENV environment") { |e|
+            ENV["RAILS_ENV"] = e
+          }
         end
 
         def parse_options(argv)

railties/test/application/test_runner_test.rb

@@ -569,6 +569,26 @@ def test_rake_passes_TESTOPTS_to_minitest
       assert_match "AccountTest#test_truth", output, "passing TEST= should run selected test"
     end
 
+    def test_running_with_ruby_gets_test_env_by_default
+      file = create_test_for_env("test")
+      results = Dir.chdir(app_path) {
+        `ruby -Ilib:test #{file}`.each_line.map { |line| JSON.parse line }
+      }
+      assert_equal 1, results.length
+      failures = results.first["failures"]
+      flunk(failures.first) unless failures.empty?
+    end
+
+    def test_running_with_ruby_can_set_env_via_cmdline
+      file = create_test_for_env("development")
+      results = Dir.chdir(app_path) {
+        `ruby -Ilib:test #{file} -e development`.each_line.map { |line| JSON.parse line }
+      }
+      assert_equal 1, results.length
+      failures = results.first["failures"]
+      flunk(failures.first) unless failures.empty?
+    end
+
     def test_rake_passes_multiple_TESTOPTS_to_minitest
       create_test_file :models, "account"
       output = Dir.chdir(app_path) { `bin/rake test TESTOPTS='-v --seed=1234'` }
@@ -727,6 +747,42 @@ def create_schema
         app_file "db/schema.rb", ""
       end
 
+      def create_test_for_env(env)
+        app_file "test/models/environment_test.rb", <<-RUBY
+          require 'test_helper'
+          class JSONReporter < Minitest::AbstractReporter
+            def record(result)
+              puts JSON.dump(klass: result.class.name,
+                             name: result.name,
+                             failures: result.failures,
+                             assertions: result.assertions,
+                             time: result.time)
+            end
+          end
+
+          def Minitest.plugin_json_reporter_init(opts)
+            Minitest.reporter.reporters.clear
+            Minitest.reporter.reporters << JSONReporter.new
+          end
+
+          Minitest.extensions << "rails"
+          Minitest.extensions << "json_reporter"
+
+          # Minitest uses RubyGems to find plugins, and since RubyGems
+          # doesn't know about the Rails installation we're pointing at,
+          # Minitest won't require the Rails minitest plugin when we run
+          # these integration tests.  So we have to manually require the
+          # Minitest plugin here.
+          require 'minitest/rails_plugin'
+
+          class EnvironmentTest < ActiveSupport::TestCase
+            def test_environment
+              assert_equal #{env.dump}, ENV["RAILS_ENV"]
+            end
+          end
+        RUBY
+      end
+
       def create_test_file(path = :unit, name = "test", pass: true)
         app_file "test/#{path}/#{name}_test.rb", <<-RUBY
           require 'test_helper'

railties/test/isolation/abstract_unit.rb

@@ -358,10 +358,12 @@ def remove_from_file(file, str)
     end
 
     def app_file(path, contents, mode = "w")
-      FileUtils.mkdir_p File.dirname("#{app_path}/#{path}")
-      File.open("#{app_path}/#{path}", mode) do |f|
+      file_name = "#{app_path}/#{path}"
+      FileUtils.mkdir_p File.dirname(file_name)
+      File.open(file_name, mode) do |f|
         f.puts contents
       end
+      file_name
     end
 
     def remove_file(path)