test: run assets variant tests in dedicated directory (#20546)

Author: sapphi-red

playground/vitestGlobalSetup.ts

@@ -43,6 +43,7 @@ export async function setup({ provide }: TestProject): Promise<void> {
     })
   // also setup dedicated copy for "variant" tests
   for (const [original, variants] of [
+    ['assets', ['encoded-base', 'relative-base', 'runtime-base', 'url-base']],
     ['css', ['lightningcss']],
     ['transform-plugin', ['base']],
   ] as const) {