Add tests for BigInt proposal

[littledan]

spec. The proposal is at Stage 2, so it would not yet be appropriate to merge, but the development of tests would help a Babel implementation and any pioneering implementations in browsers, such as Robin Templeton's in-progress work in SpiderMonkey. Robin may have tests to upstream from that effort.

[littledan]

This should be assigned to @cxielarko .

[lispwitch]

BigInt test checklist

[gsathya]

cc @jakobkummerow

[littledan]

I am not aware of testing gaps for BigInt, so closing this issue.