feat: push tests down - test more at go level

[gilescope]

These tests are currently earthbuild integration tests, this pushes them down to be go tests. Faster tests, less to go wrong.

[gemini-code-assist]

Code Review

This pull request introduces several Earthfile test fixtures to validate various valid and invalid VERSION formats, along with corresponding unit tests in ast/version_test.go. The review feedback recommends capturing the loop variables locally within the test loops before invoking parallel subtests to ensure compatibility with older Go versions and satisfy linters.

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[github-actions]

🎉 Are we earthbuild yet?

Great progress! You've reduced "earthly" occurrences by 3 (0.06%)

📈 Overall Progress

Branch	Total Count
main	4751
This PR	4748
Difference	-3 (0.06%)

📁 Changes by file type:

File Type	Change
Go files (.go)	➖ No change
Documentation (.md)	➖ No change
Earthfiles	✅ -3

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Keep up the great work migrating from Earthly to Earthbuild! 🚀

💡 Tips for finding more occurrences

Run locally to see detailed breakdown:

./.github/scripts/count-earthly.sh

Note that the goal is not to reach 0.
There is anticipated to be at least some occurences of earthly in the source code due to backwards compatibility with config files and language constructs.

[gilescope]

Had to rework it following the parsing changes.

[janishorsts]

LGTM