Reworked some of metametameta to be more flexible.

[git-p4: depot-paths = "//src/minitest/dev/": change = 12355]

test/minitest/metametameta.rb

@@ -25,6 +25,14 @@ class AnError < StandardError; include MyModule; end
 class MetaMetaMetaTestCase < Minitest::Test
   attr_accessor :reporter, :output, :tu
 
+  def with_stderr err
+    old = $stderr
+    $stderr = err
+    yield
+  ensure
+    $stderr = old
+  end
+
   def run_tu_with_fresh_reporter flags = %w[--seed 42]
     options = Minitest.process_args flags
 
@@ -34,18 +42,20 @@ def run_tu_with_fresh_reporter flags = %w[--seed 42]
     reporter << Minitest::SummaryReporter.new(@output, options)
     reporter << Minitest::ProgressReporter.new(@output, options)
 
-    reporter.start
+    with_stderr @output do
+      reporter.start
 
-    yield(reporter) if block_given?
+      yield(reporter) if block_given?
 
-    @tus ||= [@tu]
-    @tus.each do |tu|
-      Minitest::Runnable.runnables.delete tu
+      @tus ||= [@tu]
+      @tus.each do |tu|
+        Minitest::Runnable.runnables.delete tu
 
-      tu.run reporter, options
-    end
+        tu.run reporter, options
+      end
 
-    reporter.report
+      reporter.report
+    end
   end
 
   def first_reporter
@@ -84,6 +94,8 @@ def normalize_output output
       output.gsub!(/^(\s+)[^:]+:\d+:in/, '\1FILE:LINE:in')
     end
 
+    output.gsub!(/( at )[^:]+:\d+/, '\1[FILE:LINE]')
+
     output
   end