BigInt: some fixes listed in #1461 (#1485)

test/built-ins/TypedArray/prototype/fill/BigInt/fill-values-conversion-once.js
- Should refer to BigInt conversion instead of ToNumber

test/built-ins/TypedArray/prototype/fill/BigInt/fill-values-non-numeric.js
- Missing coverage for non-convertable values like undefined or null?
- added fill-values-non-numeric-throw.js

test/built-ins/TypedArray/prototype/set/BigInt/array-arg-return-abrupt-tointeger-offset-symbol.js
- Change [1] to [1n] to avoid possible false-positives because ToBigInt(1) also throws a TypeError.
- Issue also present in other set() tests.