Fix DatePeriod tests for implementing IteratorAggregate

[fluffycondor]

I wrote them in a wrong way in #8312, so they are rewritten now as @AndrolGenhald advised in #8327. But actually I'm quite surprised Psalm allows me to call getIterator() on a pure Traversable. It actually results in Fatal Error: https://3v4l.org/MEpb3 (But it's the only one way I've found to test it.) Looks like PHP 8 stubs indeed leaks to PHP 7 environment somehow.

[AndrolGenhald]

I was thinking it should be in providerInvalidCodeParse to make sure it has an error, if that test is working fine then it does seem like something is broken.

[fluffycondor]

it should be in providerInvalidCodeParse

I've tried it like that, it yields no error. I mean the test fails with the reason "No CodeException was thrown" or something. So Psalm thinks this code is valid in PHP 7.3 environment.

[weirdan]

4.x branch is closed now as we prepare for the 5.0 release. Please target the master branch instead.

[orklah]

Closing as this targets 4.x which is closed. Please reopen on master going forward