patchtest: reject Upstream-Status after scissors

patchtest: reject Upstream-Status after scissors

Upstream-Status must be placed in the patch header before the
scissors line. patchtest currently accepts tags that appear
only after the scissors, which means the tag is lost when the
patch is refreshed by git or devtool.

Update test_upstream_status_presence_format() to distinguish
between tags in the header and tags after the scissors, and
reject the latter.

Fixes: [YOCTO #15940]
Signed-off-by: Luka Krstic <lukakrstic031@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>