This fixes Vitest configs that set test.root to an absolute path.

For example, this shape was not handled correctly:

import { join } from 'node:path';
import { fileURLToPath } from 'node:url';
import { defineConfig } from 'vitest/config';

const root = join(fileURLToPath(new URL('.', import.meta.url)), 'tests');

export default defineConfig({
  test: {
    root,
    include: ['*.test.ts'],
    setupFiles: ['./setup.ts'],
  },
});

Knip resolved test.root as if it were always relative to the config file directory, so an already-absolute root ended up being joined again and produced invalid paths.

This change uses toAbsolute(...) for Vitest test.root handling in:

• config dependency resolution
• test entry path resolution
• build lib entry resolution

It also adds a regression fixture covering an absolute test.root.