test: fix weird string error

Previously getting this error when running `tap2junit` (what parses our
`.tap` files in CI):

```
Traceback (most recent call last):
  File "/usr/local/bin/tap2junit", line 11, in <module>
    sys.exit(main())
  File "/usr/local/lib/python2.7/site-packages/tap2junit/__main__.py", line 46, in main
    result.to_file(args.output, [result], prettyprint=False)
  File "/usr/local/lib/python2.7/site-packages/junit_xml/__init__.py", line 289, in to_file
    test_suites, prettyprint=prettyprint, encoding=encoding)
  File "/usr/local/lib/python2.7/site-packages/junit_xml/__init__.py", line 257, in to_xml_string
    ts_xml = ts.build_xml_doc(encoding=encoding)
  File "/usr/local/lib/python2.7/site-packages/junit_xml/__init__.py", line 221, in build_xml_doc
    attrs['message'] = decode(case.skipped_message, encoding)
  File "/usr/local/lib/python2.7/site-packages/junit_xml/__init__.py", line 68, in decode
    ret = unicode(var)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 11: ordinal not in range(128)
```

PR-URL: https://github.com/nodejs/node/pull/21793


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>