Upgrade `phpunit/phpunit` dev-dependency to 5.x

[s0600204]

The version used previously (4.8.36) was using the core php function each() which was deprecated in php 7.2 and removed for php 8.0. As such, developers running webservers with php 8 were no longer able to run tests.

As of this PR, we now rely on a slightly more recent version of phpunit (5.7.27), which also requires us to bump our minimum supported version of php to 5.6.

Refs. https://www.php.net/manual/en/function.each

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As an aside, it looks like phpunit 6.0 requires php 7.0 as a minimum - and there's probably unlikely to ever be a newer version of phpunit 5 - so this is most likely as far as we can go with phpunit and still support some flavour of php 5.

[u01jmg3]

• Do you know exactly what version of PHP 5.6.x would be needed?
• Looks like I would have to release this as v3 of the parser as we would effectively break the testing suite for those using PHP 5.3.6 to 5.5.38. I realise these are very old versions of PHP and it's hard to find stats on usage. However, this article suggests PHP 5.3 support is still provided by 8.87% of packages. Shall ponder.

[s0600204]

• Do you know exactly what version of PHP 5.6.x would be needed?

I do not, although admittedly I haven't looked further than phpunit's Packagist page (https://packagist.org/packages/phpunit/phpunit#5.7.27).

• Looks like I would have to release this as v3 of the parser

Understandable. Figured it was worth creating the PR just so that you were aware of the issue (and had a fix to hand).