move ENV["BACKTRACE"] support into the TestRunner.

railties/lib/rails/test_unit/minitest_plugin.rb

@@ -7,9 +7,8 @@ def Minitest.plugin_rails_init(options)
     options[:filter] = method
   end
 
-  if ENV["BACKTRACE"].nil? && !($rails_test_runner && $rails_test_runner.show_backtrace?)
+  if !($rails_test_runner && $rails_test_runner.show_backtrace?)
     Minitest.backtrace_filter = Rails.backtrace_cleaner
   end
 end
 Minitest.extensions << 'rails'
-

railties/lib/rails/test_unit/runner.rb

@@ -7,7 +7,7 @@ module Rails
   class TestRunner
     class Options
       def self.parse(args)
-        options = { backtrace: false, name: nil, environment: "test" }
+        options = { backtrace: !ENV["BACKTRACE"].nil?, name: nil, environment: "test" }
 
         opt_parser = ::OptionParser.new do |opts|
           opts.banner = "Usage: bin/rails test [options] [file or directory]"

railties/test/test_unit/runner_test.rb

@@ -1,7 +1,10 @@
 require 'abstract_unit'
+require 'env_helpers'
 require 'rails/test_unit/runner'
 
 class TestUnitTestRunnerTest < ActiveSupport::TestCase
+  include EnvHelpers
+
   setup do
     @options = Rails::TestRunner::Options
   end
@@ -19,6 +22,13 @@ class TestUnitTestRunnerTest < ActiveSupport::TestCase
     assert options[:backtrace]
   end
 
+  test "show full backtrace using BACKTRACE environment variable" do
+    switch_env "BACKTRACE", "true" do
+      options = @options.parse([])
+      assert options[:backtrace]
+    end
+  end
+
   test "tests run in the test environment by default" do
     options = @options.parse([])
     assert_equal "test", options[:environment]