Unit test for custom date parsing

Unit test for custom date parsing

Unit test for https://github.com/civicrm/civicrm-core/pull/14986
along with some code comments in the test about issues I hit doing what I thought was the right fix
- copied here for visibility

    // @todo I feel like we should work towards this actually parsing $params here -
    // & dropping formatting but
    // per https://github.com/civicrm/civicrm-core/pull/14986 for now $formatted is parsing
    // The issue I hit was that when I tried to extend to checking they were correctly imported
    // I was not actually sure what correct behaviour was for what dates were accepted since
    // it seems to ignore the latter in favour of the former - which seems wrong.