add debug print for the failure

http://ci.rvm.jp/results/trunk-gc-asserts@ruby-sp2-docker/4364584

```
  1) Failure:
TestObjSpace#test_dump_all [/tmp/ruby/src/trunk-gc-asserts/test/objspace/test_objspace.rb:599]:
number of strings.
<2> expected but was
<3>.
```

1 files changed, 8 insertions, 1 deletions

diff --git a/test/objspace/test_objspace.rb b/test/objspace/test_objspace.rb
index f0f294c31a..32cd1eda24 100644
--- a/test/objspace/test_objspace.rb
+++ b/test/objspace/test_objspace.rb
@@ -580,7 +580,14 @@ class TestObjSpace < Test::Unit::TestCase
     # This test makes assertions on the assignment to `str`, so we look for
     # the second appearance of /TEST STRING/ in the output
     test_string_in_dump_all = output.grep(/TEST2/)
-    assert_equal(2, test_string_in_dump_all.size, "number of strings")
+
+    begin
+      assert_equal(2, test_string_in_dump_all.size, "number of strings")
+    rescue Exception => e
+      STDERR.puts e.inspect
+      STDERR.puts test_string_in_dump_all
+      raise
+    end
 
     entry_hash = JSON.parse(test_string_in_dump_all[1])