oe-selftest: devtool: test modify git recipe building from a subdir

oe-selftest: devtool: test modify git recipe building from a subdir

Add a test that verifies that devtool modify + devtool finish do the
right thing on a recipe that fetches from git and sets S to point to
a subdirectory of the source tree. We have a few examples among the core
recipes, dos2unix is a convenient one so let's use that. (The test first
verifies that that is still true in case the recipe is changed in
future.)

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