Plugin Dependencies: Add a test to ensure that hello.php is converted to hello-dolly.

[costdev]

Hello Dolly requires special handling as it is, by default, a single file plugin called hello.php. However, when installed via the Plugins Repository, it is placed in a nested directory called hello-dolly. The correct slug for Hello Dolly is therefore hello-dolly, not hello.

Recent commits to Plugin Dependencies added this special handling which exists elsewhere in Core which was manually tested.

This PR adds a PHPUnit unit to verify and protect the behaviour.

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