WIP: Test262 support

[o-]

Add support for running Test262. To avoid polluting the outer environment the tests run in an iframe.

To do so we add:

• A new minimal inner testharness-client which can collect Test262 results and post them to the parent site.
• A test262 specific outer testharness which receives the results and reports them using the normal mechanism.
• A harness-adapter which provides the minimally required infrastructure test262 tests expect from the global environment.

TBD: Currently the code assumes Test262 to be checked out in the root of the repo. It should be added as a dependency somehow.