tool/lib/test/unit/parallel.rb: fail explicitly when failing to get io

`(ulimit -n 30; make test-tool)` fails with unexplicit message:
"undefined method `write' for nil:NilClass" due to lack of stdout.

This change makes it explicit.  [Bug #5577]

tool/lib/test/unit/parallel.rb

@@ -145,9 +145,13 @@ module Test
         rescue Exception => e
           begin
             trace = e.backtrace || ['unknown method']
-            err = ["#{trace.shift}: #{e.message} (#{e.class})"] + trace.map{|t| t.prepend("\t") }
+            err = ["#{trace.shift}: #{e.message} (#{e.class})"] + trace.map{|t| "\t" + t }
 
-            _report "bye", Marshal.dump(err.join("\n"))
+            if @stdout
+              _report "bye", Marshal.dump(err.join("\n"))
+            else
+              raise "failed to report a failure due to lack of @stdout"
+            end
           rescue Errno::EPIPE;end
           exit
         ensure

tool/test/testunit/test_parallel.rb

@@ -98,6 +98,7 @@ module TestParallel
         while buf = @worker_out.gets
           break if /^p (.+?)$/ =~ buf
         end
+        assert_not_nil($1, "'p' was not found")
         assert_match(/TestA#test_nothing_test = \d+\.\d+ s = \.\n/, $1.chomp.unpack("m")[0])
       end
     end
@@ -108,6 +109,7 @@ module TestParallel
         while buf = @worker_out.gets
           break if /^done (.+?)$/ =~ buf
         end
+        assert_not_nil($1, "'done' was not found")
 
         result = Marshal.load($1.chomp.unpack("m")[0])
         assert_equal(5, result[0])