Add e2e tests

[reimic]

🚧 Work in progress 🚧

What does this PR do?

• adds e2e tests

todo:

• set phpunit version to 6.5
• fix all unit tests to match phpunit == 6.5
• fix issues uncovered in tests
• examine why a test failing validation is displayed as passing
• fix assertions

[reimic]

@adamziel it would be cool if you could take a look at supposedly passing tests for PHP == 8.3 (hint - they do not pass):

The CI test Running json blueprint with steps fails at step setSiteOptions, which is quite a surprise. Locally it fails at defineWpConfigConsts which indicates a Windows compatibility issue, again.

The CI test Running json blueprint with word press version fails only because I did not properly prepare what should be asserted. But locally I get an error for step downloadWordPress. Again, most likely a Windows problem...

For php blueprints - all fail at validation. I'll make an issue for that.... and I'll add another one to enhance the quality of errors thrown.

Array
(
    [/constants] => Array
        (
            [0] => The data (array) must match the type: object
        )

)

That's not the most informative thing to get from a validator. We can do better.

[adamziel]

Let's chat in https://github.com/WordPress/blueprints-library/issues/83