tests load user's configuration

I stumbled upon this because I had setup a demo with a config file which relocated my environment, and this caused tests to break when running tests on master with the following error:

self = <buildstream._context.Context object at 0x7f5310deb978>, config = '/home/tristan/.config/buildstream.conf'

    def load(self, config=None):
        profile_start(Topics.LOAD_CONTEXT, 'load')
    
        # If a specific config file is not specified, default to trying
        # a $XDG_CONFIG_HOME/buildstream.conf file
        #
        if not config:
            default_config = os.path.join(os.environ['XDG_CONFIG_HOME'],
                                          'buildstream.conf')
            if os.path.exists(default_config):
                config = default_config
    
        # Load default config
        #
        defaults = _yaml.load(_site.default_user_config)
    
        if config:
            self.config_origin = os.path.abspath(config)
            user_config = _yaml.load(config)
            _yaml.composite(defaults, user_config)
    
        # Give obsoletion warnings
        if defaults.get('builddir'):
            raise LoadError(LoadErrorReason.INVALID_DATA,
>                           "builddir is obsolete, use cachedir")
E           buildstream._exceptions.LoadError: builddir is obsolete, use cachedir

buildstream/_context.py:202: LoadError

The problem is simply that we are not overriding the XDG_ family of environment variables for tests, so the tests can be affected by the host environment.