Add self-tests for the Validation test

[Luc45]

This PR adds self-tests for the Validation test.

We added 4 scenarios:

• Main plugin
• Missing plugin
• Main theme
• Missing theme

Then we generated the snapshots of the results to make sure they are as expected.

Testing Instructions

• Just review the snapshots and make sure they are as expected
• The ones with "Main" should be a success and the PHP header should be correctly identified
• The ones with "Missing" should be a warning, and the PHP header should be false

Examples:

[singerb]

Snapshots look correct, tests run correctly.

[Luc45]

Sounds good, I'll be merging this when the upstream PR has been merged.