tests: move background-shorthand test to background-shorthand-single

And update/fix it for the new shorthand parsing, since the old test did
a couple of things wrong:
- it erroneously flipped background-clip and background-origin when
  testing
- it was missing default initial values for background-size and
  background-position from the generated test output

tests/css/parser/Makefile.am

@@ -154,8 +154,8 @@ EXTRA_DIST += \
 	background-position.ref.css \
 	background-repeat.css \
 	background-repeat.ref.css \
-	background-shorthand.css \
+	background-shorthand-single.css \
-	background-shorthand.ref.css \
+	background-shorthand-single.ref.css \
         background-size.css \
         background-size.ref.css \
 	boolean.css \