Test string whitespace coercion for BigInt values

[leobalter]

follow up for #1349

Some related files:

• test/built-ins/BigInt/constructor-empty-string.js
• test/built-ins/BigInt/constructor-trailing-leading-spaces.js

[linusg]

Five years later I've just fixed a bug in our engine that would've been caught by these tests, maybe someone still wants to add them for future implementors :^)