PHP 8.5 | Tests: prevent deprecation notice for Reflection*::setAccessible()

[jrfnl]

Description

Since PHP 8.1, calling the Reflection*::setAccessible() methods is no longer necessary as reflected properties/methods/etc will always be accessible. However, the method calls are still needed for PHP < 8.1.

As of PHP 8.5, calling the Reflection*::setAccessible() methods is now formally deprecated and will yield a deprecation notice, which will fail test runs. As of PHP 9.0, the setAccessible() method(s) will be removed.

With the latter in mind, this commit prevents the deprecation notice by making the calls to setAccessible() conditional.

Silencing the deprecation would mean, this would need to be "fixed" again come PHP 9.0, while the current solution should be stable, including for PHP 9.0.

Ref: https://wiki.php.net/rfc/deprecations_php_8_5#extreflection_deprecations

Suggested changelog entry

N/A: test only change

[jrfnl]

Notes:

1. This PR is not claiming to fix all PHP 8.5 runtime deprecations - not all PRs have been merged into PHP Core yet, so there may be more issues which need fixing.
2. There will currently be one remaining test failure. This is not something we need to fix, but was a snafu in PHP Core itself, which has been fixed in the mean time, but the setup-php PHP 8.5 image only updates once every 24 hours, so the failure should be gone once the image has been re-generated.