Revert "refactor: don't expose Deno[Deno.internal].core namespace" (#18881)

Also conditionally disabled one test if there's not enough space on
device.

test_napi/cleanup_hook_test.js

@@ -12,7 +12,6 @@ if (import.meta.main) {
     const { stdout, stderr, code } = await new Deno.Command(Deno.execPath(), {
       args: [
         "run",
-        "--enable-testing-features-do-not-use",
         "--allow-read",
         "--allow-run",
         "--allow-ffi",