oe-selftest-image: Ensure the image has sftp as well as dropbear

We need sftp so that scp works with recent openssh. Use the packagegroup
instead of a direct dependency to ensure this.

(From OE-Core rev: 70b3c7004e8f14a99adf6119f153a08ec4a4bc6b)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 2b76c8e5fc8802bbe54371119e6bf6312bf2a8ec)
Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-selftest/recipes-test/images/oe-selftest-image.bb

@@ -1,6 +1,6 @@
 SUMMARY = "An image used during oe-selftest tests"
 
-IMAGE_INSTALL = "packagegroup-core-boot dropbear"
+IMAGE_INSTALL = "packagegroup-core-boot packagegroup-core-ssh-dropbear"
 IMAGE_FEATURES = "debug-tweaks"
 
 IMAGE_LINGUAS = " "