Improve gigasecond tests (#289)
Improve gigasecond tests (#289)

* Add assertions that returned object is instance of DateTimeImmutable
Removed testNoMutation() - this is no longer valid because of the replacement of DateTime with DateTimeImmutable

* Renamed test method names to match problem specification description
See https://github.com/exercism/problem-specifications/blob/master/exercises/gigasecond/canonical-data.json

* Unify used date format

* Safely us single quotes instead

* Fix typo

* Update instructions in comment.
Add a more meaningful message to markTestSkipped

* Remove empty space before return type declaration colon
Remove useless testYourself()

* Add single space before colon in return type declaration

* Add dataProvider to reduce redundancy

exercism/xphp,exercism/xphp