oe-selftest: devtool: test that updating a file with subdir= works

oe-selftest: devtool: test that updating a file with subdir= works

If you have a file:// entry in SRC_URI with a subdir= parameter that
makes it extract into the source tree, then when you update that file in
oe-local-files and run devtool update-recipe then you want the original
file to be updated. This was made to work by OE-Core commit
9069fef5dad5a873c8a8f720f7bcbc7625556309 together with
31f1bbad248c36a8c86dde4ff57ce42efc664082, however until now there was no
oe-selftest test to verify it.

Note that in order to succeed this test also requires the fix
"lib/oe/recipeutils: ignore archives by default in
get_recipe_local_files()" since the test recipe uses a local tarball.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>