patch 8.2.1593: tests do not check the error number properly

Problem:    Tests do not check the error number properly.0
Solution:   Add a colon after the error number. (closes #6869)
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index ec4898f..a92dc25 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -976,11 +976,11 @@
 func Test_win_execute_closing_curwin()
   split
   let winid = popup_create('some text', {})
-  call assert_fails('call win_execute(winid, winnr() .. "close")', 'E994')
+  call assert_fails('call win_execute(winid, winnr() .. "close")', 'E994:')
   call popup_clear()
 
   let winid = popup_create('some text', {})
-  call assert_fails('call win_execute(winid, printf("normal! :\<C-u>call popup_close(%d)\<CR>", winid))', 'E994')
+  call assert_fails('call win_execute(winid, printf("normal! :\<C-u>call popup_close(%d)\<CR>", winid))', 'E994:')
   call popup_clear()
 endfunc