[spec] GC objects no-op prevent extensions

[mhofman]

Fixes #2008

[WebReflection]

LGTM but if we could add:

test(() => {
  const struct = functions.makeStruct();
  const array = functions.makeArray();
  assert_true((() => { for (const _ in struct); return true; })());
  assert_true((() => { for (const _ in array); return true; })());
}, "for...in");

then we'd be more covered around the current state, imho.

[tlively]

This change to intentionally-designed behavior should be discussed with the CG first. Let's keep further discussion on #2008.