If you have the string /usr/bin our YAML serializer will not quote this, but if you have the string C:\foo\bar it will quote it.
While technically not required by the YAML standard, it's nice if we're consistent on quoting them since / and \ both commonly appear in paths. Otherwise, the same YAML value may be quoted or unquoted depending on what platform a test is run on. This makes for confusing FileCheck statements.