ui: Test for abstract_ui whenever a minimal t_colors value is required

t_colors should not be checked when abstract_ui is active, because nvim UI is
not limited to a terminal.

src/nvim/edit.c

@@ -2294,8 +2294,7 @@ static int pum_wanted(void)
     return FALSE;
 
   /* The display looks bad on a B&W display. */
-  if (t_colors < 8
-      )
+  if (!abstract_ui && t_colors < 8)
     return FALSE;
   return TRUE;
 }