[TASK] Use PHPUnit 9.x with PHP 7.3+

[JakeQZ]

This is required to be able to test against PHP 8. However, PHPUnit 9.x
requires PHP 7.3, so a different version of PHPUnit is required for different
testing environments.

The main change here is a step in the GitHub Action to conditionally update
PHPUnit via PHIVE for PHP >=7.3. PHIVE does not yet have the ability for
conditional installs (see
phar-io/phive#295 (comment)) so the
script must check the PHP version before running the update.

PHIVE has also been added to the tools (self-referencing) as it is not available
by default to GitHub Actions.

Note: There are warnings from PHPUnit 9.x about use of deprecated assert
methods (which will be removed in PHPUnit 10.x). However, these don't cause the
tests to fail, and the replacement methods are not available in PHPUnit 7.x
which is still required to test against PHP 7.1 and 7.2.

Part of #925/#926.

[JakeQZ]

Managed to add --no-progress (it seems the options must come first on the command line), but it seems --trust-gpg-keys is not supported in PHIVE version 0.13.5 (I get 'unknown option' whether it's first or last on the command line)?

Where is the documentation for PHIVE?

[oliverklee]

Where is the documentation for PHIVE?

It's built-in: #930 (comment)

[oliverklee]

Please let's use the latest PHIVE version and explicitly allow Sebastian's GPG key.

[JakeQZ]

explicitly allow Sebastian's GPG key.

This does not appear to be possible. The --trust-gpg-keys option is only available with the install command, not with the update command. With the install command, it does not seem possible to specify that the PHAR file be copied with a .phar extension (it is copied with no file extension, as simply e.g. phpunit), nor any version constraints - any pre-existing entry in phive.xml is simply ignored and overwritten.

Some proper documentation for PHIVE would be really helpful.

[oliverklee]

Okay, then it seems we need to keep this workaround for trusting the GPG key.

Please let's still use the latest PHIVE version.

[JakeQZ]

I've added phar-io/phive#296.

[JakeQZ]

Please let's still use the latest PHIVE version.

Done.

[oliverklee]

I think #906 has introduced some changes that are not compatible with PHPUnit 9.

[oliverklee]

PHPUnit's Constraint parent class does not have a constructor in PHPUnit 9.x anymore. So maybe we can copy the old version's constructor to our CssConstraint class:

    public function __construct()
    {
        $this->exporter = new Exporter();
    }

(And create an issue that we should rework this once we're PHPUnit-9-only.)

[JakeQZ]

(And create an issue that we should rework this once we're PHPUnit-9-only.)

I've added a constructor which will conditionally call the parent constructor only if it exists. This should cover all bases, including if some future version of PHPUnit reintroduces a constructor. So I don't think any future rework would be required.

I can't help thinking that PHP should provide an implicit default constructor, as C++ does, to avoid this type of issue.

[oliverklee]

<3

[JakeQZ]

There are some suggestions for how to handle PHIVE in relation to this in the responses to phar-io/phive#296. I don't know if this is something we want to address in the short term; it will become a non-issue when we drop PHP 7.2 support.