BACKTRACE environment variable to show unfiltered backtraces.

We used to support the `BACKTRACE` environment variable but when
we switched to MiniTest it got removed: f9382cd7

This commit adds back the functionality to show the unfiltered backtrace when needed.
This also works when you run your tests with `rake`:

* `BACKTRACE=1 bin/rake test`
* `BACKTRACE=1 ruby -Itest ...`

railties/CHANGELOG.md

+*   `BACKTRACE` environment variable to show unfiltered backtraces for
+    test failures.
+
+    Example:
+
+        `BACKTRACE=1 ruby -Itest ...`
+        # or with rake
+        `BAKCTRACE=1 bin/rake`
+
+    *Yves Senn*
+
 *   Removal of all javascript stuff (gems and files) when generating a new
     application using the `--skip-javascript` option.
 

railties/lib/rails/test_help.rb

@@ -10,7 +10,9 @@
 
 # Config Rails backtrace in tests.
 require 'rails/backtrace_cleaner'
-MiniTest.backtrace_filter = Rails.backtrace_cleaner
+if ENV["BACKTRACE"].nil?
+  MiniTest.backtrace_filter = Rails.backtrace_cleaner
+end
 
 if defined?(ActiveRecord::Base)
   class ActiveSupport::TestCase

railties/test/application/test_test.rb

@@ -24,7 +24,7 @@ def test_truth
         end
       RUBY
 
-      run_test_file 'unit/foo_test.rb'
+      assert_successful_test_run 'unit/foo_test.rb'
     end
 
     test "integration test" do
@@ -49,19 +49,48 @@ def test_index
         end
       RUBY
 
-      run_test_file 'integration/posts_test.rb'
+      assert_successful_test_run 'integration/posts_test.rb'
+    end
+
+    test "enable full backtraces on test failures" do
+      app_file 'test/unit/failing_test.rb', <<-RUBY
+        require 'test_helper'
+
+        class FailingTest < ActiveSupport::TestCase
+          def test_failure
+            raise "fail"
+          end
+        end
+      RUBY
+
+      output = run_test_file('unit/failing_test.rb', env: { "BACKTRACE" => "1" })
+      assert_match %r{/app/test/unit/failing_test\.rb}, output
     end
 
     private
-      def run_test_file(name)
-        result = ruby '-Itest', "#{app_path}/test/#{name}"
+      def assert_successful_test_run(name)
+        result = run_test_file(name)
         assert_equal 0, $?.to_i, result
       end
 
+      def run_test_file(name, options = {})
+        ruby '-Itest', "#{app_path}/test/#{name}", options
+      end
+
       def ruby(*args)
+        options = args.extract_options!
+        env = options.fetch(:env, {})
+        env["RUBYLIB"] = $:.join(':')
+
         Dir.chdir(app_path) do
-          `RUBYLIB='#{$:.join(':')}' #{Gem.ruby} #{args.join(' ')}`
+          `#{env_string(env)} #{Gem.ruby} #{args.join(' ')}`
         end
       end
+
+      def env_string(variables)
+        variables.map do |key, value|
+          "#{key}='#{value}'"
+        end.join " "
+      end
   end
 end