Update recently-added PathUrlTest to be more representative

Update recently-added PathUrlTest to be more representative

Before
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The test generates the URL for an example page (`civicrm/mailing/subscribe`)
by calling `url()`. It does not explicitly set the `url(...$frontend...)` option,
so it defaults to `FALSE`.

By contrast, the other places which make URLs for `civicrm/mailing/subscribe`
(eg `CRM/Mailing/BAO/Mailing.php` and `CRM/Utils/Token.php`) do set `$frontend` to `TRUE`.

After
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The test behaves the same way as other code.