Test262Error objects have no stack trace

[thejoshwolfe]

(Tested in v8.)

When an assertion throws a Test262Error, there's no stack trace printed. If I change the assert implementation to throw an Error instead, I get a stack trace as expected.

I understand that a stack trace is not part of the ecma262 spec, so it's not a primary concern for test262, but it would be nice.

I played around with making a pull request that fixes this issue, but it seems pretty difficult to get this to work without mentioning any implementation-specific stack field and without using ES6 classes.