Add Behat step for installing WordPress with specific version #555

Job	Run time
code-quality / Lint GitHub Actions workflows	9s
code-quality / Lint PHP files	23s
code-quality / Lint Gherkin Feature files	20s
code-quality / PHPStan	21s
code-quality / PHPCS	24s
code-quality / Spell check	5s
	1m 42s

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