oeqa/selftest/tinfoil: Add test for separate config_data with recipe_parse_file()

We've seen two different regressions in this API since it is used by
layer-index but not be the core code. Add a test for it to try and
ensure we don't break it again.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>