Add stderr to output on test failure

When using waf-unittest in Jubatus development, we found that test processes sometimes fail silently without error message, as shown below:

http://ci.jubat.us/job/develop_configure/850/testReport/junit/%28root%29/configure/__enable_zookeeper___disable_eigen___enable_re2___enable_mecab___enable_ux/#footer

We found that gtest sometimes fails to catch exceptions raised in multi-threaded test code. To mitigate the situation, I propose to output stderr in addition to stdout. In the above case, stderr may contain important message to investigate the cause of the problem, like this:

```
terminate called after throwing an instance of 'mp::system_error'
  what():  socket() failed: Too many open files
```

Related to: https://github.com/jubatus/jubatus/issues/726