MAINT Unvendor tests at the lockfile creation step

[ryanking13]

This PR changes when test codes are unvendored.

Before:

• it is unvendored right after the wheel file is built (== at the same step when build.post script runs).
• when generating a lockfile, we check if the test codes are unvendored and update the lockfile entry.

After:

• it is unvendored when we are creating a lockfile.

Why?

I would like to move the unvendoring logic to somewhere else like pyodide-pack or pyodide-lock (pyodide/pyodide-lock#30, pyodide/pyodide-pack#41), so package maintainers can use that feature when they are not using a recipe.

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Related issues:

• Move test file unvendoring functionality from pyodide-build to pyodide-lock pyodide-lock#30
• Add test files unvendoring functionality pyodide-pack#41