Bubble assertion failures back to the main thread

Without some help, failures in a forked process make for some noise in
the output, but won't fail the build.

Instead of trying to transfer the whole exception back, I've gone for a
simpler solution of just sending _something_ (the exception class name)
back so we'll fail; the full failure will be visible in the child
process's stderr output.

activesupport/test/evented_file_update_checker_test.rb

@@ -55,6 +55,7 @@ class EventedFileUpdateCheckerTest < ActiveSupport::TestCase
     # Pipes used for flow control across fork.
     boot_reader, boot_writer = IO.pipe
     touch_reader, touch_writer = IO.pipe
+    result_reader, result_writer = IO.pipe
 
     pid = fork do
       assert_not_predicate checker, :updated?
@@ -68,10 +69,17 @@ class EventedFileUpdateCheckerTest < ActiveSupport::TestCase
       IO.select([touch_reader])
 
       assert_predicate checker, :updated?
+    rescue Exception => ex
+      result_writer.write(ex.class.name)
+      raise
+    ensure
+      result_writer.close
     end
 
     assert pid
 
+    result_writer.close
+
     # Wait for fork to be booted before touching files.
     IO.select([boot_reader])
     touch(tmpfiles)
@@ -82,6 +90,8 @@ class EventedFileUpdateCheckerTest < ActiveSupport::TestCase
     assert_predicate checker, :updated?
 
     Process.wait(pid)
+
+    assert_equal "", result_reader.read
   end
 
   test "can be garbage collected" do