test/parser: Check for full URLs in parser tests

At present, we only match the 'file' portion of a URL in the parser
tests, so we "serve" a file if just the filename (but not the scheme,
hostname or path) matches the file we set with test_read_conf_embedded.

This change introduces test_read_conf_embedded_url, which we can use to
specify a full URL. In this case, the fake parser_request_file matches
the entire URL before returning the file data.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>