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: 2b76c8e5fc8802bbe54371119e6bf6312bf2a8ec)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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

@@ -1,7 +1,7 @@
 SUMMARY = "An image used during oe-selftest tests"
 
 # libudev is needed for deploy mdadm via devtool
-IMAGE_INSTALL = "packagegroup-core-boot dropbear libudev"
+IMAGE_INSTALL = "packagegroup-core-boot packagegroup-core-ssh-dropbear libudev"
 IMAGE_FEATURES = "debug-tweaks"
 
 IMAGE_LINGUAS = " "