Test for TypedArray.prototype.fill semantics change (#927)

The change is proposed in https://github.com/tc39/ecma262/pull/856
as a fix to https://github.com/tc39/ecma262/issues/855

Here, the ToNumber coercion is done only once, rather than on each
iteration. It does not appear that there were previously any
tests against repeated coercion for this parameter previously.

Tested this test against V8, which failed, as V8 implements the
current spec rather than the proposed one.