Strengthen test_show_cert_form_confirm to assert result.days

A blank/default days value round-trips as "" even from a reverted
three-field form (CertFormResult.days defaults to ""), so it would not
prove the Days field was traversed. Use a non-blank days="90" instead,
which only survives if the field exists and is validated by _submit().

tests/test_dialogs.py

@@ -116,16 +116,23 @@ def test_show_cert_form_uses_erase_not_clear():
 
 def test_show_cert_form_confirm():
     """Enter advances through the 4 fields to the Continue button; Enter on
-    Continue confirms.  That's 5 Enter keypresses total."""
+    Continue confirms.  That's 5 Enter keypresses total.
+
+    days="90" (non-blank) is used deliberately: a blank default would still
+    read back as "" even from a reverted three-field form (CertFormResult.days
+    defaults to ""), so it would not actually prove the days field was
+    traversed. A non-blank value only round-trips if the days field exists,
+    was reached by the Enter sequence, and was carried into the result."""
     stdscr = _make_stdscr()
     win = _make_win(rows=20, cols=70)
     win.getch.side_effect = [10, 10, 10, 10, 10]
     with patch("curses.newwin", return_value=win):
-        result = show_cert_form(stdscr, cn="bob", email="bob@example.com")
+        result = show_cert_form(stdscr, cn="bob", email="bob@example.com", days="90")
     assert isinstance(result, CertFormResult)
     assert result.confirmed is True
     assert result.cn == "bob"
     assert result.email == "bob@example.com"
+    assert result.days == "90"
     assert len(result.password) > 0