test_function.rb: fix messages

* test/fiddle/test_function.rb (test_nogvl_poll): fix messages as
  failed conditions, with errno description.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64091 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/fiddle/test_function.rb

@@ -86,15 +86,26 @@ def test_nogvl_poll
       n1 = f.call(nil, 0, msec)
       n2 = th.value
       t1 = Process.clock_gettime(Process::CLOCK_MONOTONIC, :millisecond)
-      assert_in_delta(msec, t1 - t0, 100, 'slept correct amount of time')
-      assert_equal(0, n1, 'poll(2) called correctly main-thread')
-      assert_equal(0, n2, 'poll(2) called correctly in sub-thread')
+      assert_in_delta(msec, t1 - t0, 100, 'slept amount of time')
+      assert_equal(0, n1, perror("poll(2) in main-thread"))
+      assert_equal(0, n2, perror("poll(2) in sub-thread"))
     end
 
     def test_no_memory_leak
       prep = 'r = Fiddle::Function.new(Fiddle.dlopen(nil)["rb_obj_tainted"], [Fiddle::TYPE_UINTPTR_T], Fiddle::TYPE_UINTPTR_T); a = "a"'
       code = 'begin r.call(a); rescue TypeError; end'
       assert_no_memory_leak(%w[-W0 -rfiddle], "#{prep}\n1000.times{#{code}}", "10_000.times {#{code}}", limit: 1.2)
     end
+
+    private
+
+    def perror(m)
+      proc do
+        if e = Fiddle.last_error
+          m = "#{m}: #{SystemCallError.new(e).message}"
+        end
+        m
+      end
+    end
   end
 end if defined?(Fiddle)