patch 9.0.0458: splitting a line with a text prop "above" moves it down

Problem:    Splitting a line with a text prop "above" moves it to a new line
            below.
Solution:   Keep an "above" text prop above the first line.
diff --git a/src/testdir/test_textprop.vim b/src/testdir/test_textprop.vim
index 235dd02..5167e8e 100644
--- a/src/testdir/test_textprop.vim
+++ b/src/testdir/test_textprop.vim
@@ -2886,6 +2886,9 @@
   call term_sendkeys(buf, "V")
   call VerifyScreenDump(buf, 'Test_prop_with_text_above_7', {})
 
+  call term_sendkeys(buf, "\<Esc>ls\<CR>\<Esc>")
+  call VerifyScreenDump(buf, 'Test_prop_with_text_above_8', {})
+
   call StopVimInTerminal(buf)
 endfunc