Make better use of Minitest's lifecycle

http://blog.arkency.com/2013/06/are-we-abusing-at-exit/

test/capture_warnings.rb

@@ -10,49 +10,56 @@ class CaptureWarnings
     @output_dir = File.join(app_root, 'tmp')
     FileUtils.mkdir_p(output_dir)
     @bundle_dir = File.join(app_root, 'bundle')
+    @output = STDOUT
   end
 
-  def before_tests
-    $stderr.reopen(stderr_file.path)
+  def execute!
     $VERBOSE = true
-    at_exit { $stderr.reopen(STDERR) }
+    $stderr.reopen(stderr_file.path)
+
+    Minitest.after_run do
+      stderr_file.rewind
+      lines = stderr_file.read.split("\n")
+      stderr_file.close!
+      $stderr.reopen(STDERR)
+      after_tests(lines)
+    end
   end
 
-  def after_tests
-    stderr_file.rewind
-    lines = stderr_file.read.split("\n")
-    stderr_file.close!
-
-    $stderr.reopen(STDERR)
-
+  # rubocop:disable Metrics/AbcSize
+  def after_tests(lines)
     app_warnings, other_warnings = lines.partition { |line|
       line.include?(app_root) && !line.include?(bundle_dir)
     }
 
+    header = "#{'-' * 22} app warnings: #{'-' * 22}"
+    output.puts
+    output.puts header
+
     if app_warnings.any?
-      puts <<-WARNINGS
-#{'-' * 30} app warnings: #{'-' * 30}
-
-#{app_warnings.join("\n")}
-
-#{'-' * 75}
-      WARNINGS
+      output.puts app_warnings.join("\n")
+    else
+      output.puts 'None. Yay!'
     end
 
     if other_warnings.any?
       File.write(File.join(output_dir, 'warnings.txt'), other_warnings.join("\n") << "\n")
-      puts
-      puts 'Non-app warnings written to tmp/warnings.txt'
-      puts
+      output.puts
+      output.puts 'Non-app warnings written to tmp/warnings.txt'
+      output.puts
     end
 
+    output.puts
+    output.puts '-' * header.size
+
     # fail the build...
     if fail_on_warnings && app_warnings.any?
       abort "Failing build due to app warnings: #{app_warnings.inspect}"
     end
   end
+  # rubocop:enable Metrics/AbcSize
 
   private
 
-  attr_reader :stderr_file, :app_root, :output_dir, :bundle_dir, :fail_on_warnings
+  attr_reader :stderr_file, :app_root, :output_dir, :bundle_dir, :fail_on_warnings, :output
 end