patch 9.0.0652: 'smoothscroll' not tested with 'number' and "n" in 'cpo'

Problem:    'smoothscroll' not tested with 'number' and "n" in 'cpo'.
Solution:   Add tests, fix uncovered problem.
diff --git a/src/drawline.c b/src/drawline.c
index 4d504e1..d36eee7 100644
--- a/src/drawline.c
+++ b/src/drawline.c
@@ -347,8 +347,9 @@
 	int		num_attr UNUSED)
 {
     if ((wp->w_p_nu || wp->w_p_rnu)
-	    && (wlv->row == wlv->startrow + wlv->filler_lines
-			 || vim_strchr(p_cpo, CPO_NUMCOL) == NULL))
+	    && ((wlv->row == wlv->startrow + wlv->filler_lines
+		    && (wp->w_skipcol == 0 || wlv->row > wp->w_winrow))
+		|| vim_strchr(p_cpo, CPO_NUMCOL) == NULL))
     {
 #ifdef FEAT_SIGNS
 	// If 'signcolumn' is set to 'number' and a sign is present