patch 8.1.1811: popup window color cannot be set to "Normal"

Problem:    Popup window color cannot be set to "Normal".
Solution:   Check for non-empty 'wincolor' instead of zero attribute.
            (closes #4772)
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 5a841e3..8a58926 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -86,7 +86,7 @@
 	  call setline(1, range(1, 100))
 	  call popup_create('hello border', #{line: 2, col: 3, border: []})
 	  call popup_create('hello padding', #{line: 2, col: 23, padding: []})
-	  call popup_create('hello both', #{line: 2, col: 43, border: [], padding: []})
+	  call popup_create('hello both', #{line: 2, col: 43, border: [], padding: [], highlight: 'Normal'})
 	  call popup_create('border TL', #{line: 6, col: 3, border: [1, 0, 0, 4]})
 	  call popup_create('paddings', #{line: 6, col: 23, padding: [1, 3, 2, 4]})
 	  call popup_create('wrapped longer text', #{line: 8, col: 55, padding: [0, 3, 0, 3], border: [0, 1, 0, 1]})