oeqa/selftest/devtool: add test for modifying recipes using go.bbclass

go.bbclass uses a special do_unpack function that causes the git root
to be different from S.  Verify that it unpacks as expected.

[ YOCTO #15483 ]

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