[js_runtime] corelib{,_2}/growable_list_test - `JSArray.addAll` checks ConcurrentModificationError in assert

[rakudrama]

JSArray.addAll checks for a ConcurrentModificationError in a assert.

As a consequence, the tests

corelib_2/growable_list_test RuntimeError
corelib/growable_list_test RuntimeError

are failing on configurations that run without asserts enabled.

I think it is reasonable for a CME check for an unlikely case to be checked in an assertion rather than in production.
We should assess if moving the CME check to an assert was beneficial for performance, and whether we have good test coverage for assert-based checks.

[fishythefish]

Perhaps we should consider having these checks enabled by default and disabling them with an appropriate flag/optimization level. I think we should aim to be compliant by default and not rely on assertions being enabled - plus it feels weird to assert an invariant we do not establish/maintain.