Fix PHPUnit tests

[edegaudenzi]

This issue tries to respond to this, basically to fix unit tests.

I think I have done all the necessary, or at least all the basics to make them run.
I've also added coverage annotations - to avoid risk errors - using the new (PHPUnit 10+) attributes syntax.

Branch I've got at the moment runs PHPUnit v12 (current) on its PHP 8.3+ requirement.
However, I've just tried on previous versions of PHPUnit and I've successfully run the very same tests on PHPUnit version 10 on its PHP 8.1+ requirement as mentioned.

Taking in consideration PHP Supported Versions - long-story-short: 8.1 deprecated, 8.2 about to be - let me know what's the flavour you want. My suggestion would be PHPUnit v12 (requires PHP 8.3+).

Note for @calcinai :
Unit Tests highlighted a legit fail, whereby one of the assertions in XeroPHP\Tests\Remote\ModelTest::testPracticeManagerClientListIsReturned, particularly $this->assertTrue($contact->getIsPrimary());, returns 'Yes' instead of true.
I have a feeling the assertion expecting true is the correct answer, obtainable by simply filter_var('Yes', FILTER_VALIDATE_BOOLEAN) (I love you php!). I can submit another PR to fix either the test or the code.

[bretto36]

@edegaudenzi the assertTrue for getIsPrimary should technically return 'Yes' not true, I added that code originally and was trying to be clever to make it a boolean, but it should just return the underlying value from the XPM Api which is a string, i had a PR in that took everything to v8.1 compatible - #921

but there are also PR's for #925 to keep php v5.6 so i think calcinai is in a tricky place trying to keep everyone happy.

I agree with you in that we need to be able to support a PHP version that is current though.