WPT: Run tests with unsafeEval so they can use IOContext

[npaun]

WPT tests are now declared as text bindings and executed with unsafeEval. This way they can do IOContext things.
This PR also fixes promise_test to store each promise created and await them all before evaluating the test results.

[anonrig]

What does this PR does that we couldn't do in the past? Can you add those changes to this pull-request as well. It's not possible to justify these changes without an example/test that comes with it.

[jasnell]

Many of the wpt test files rely on the need to perform io at the top level scope, as you know we want to be able to run those wpt files automatically as is, without porting those to workerd syntax. This PR is the solution @npaun and I came up with to support the case. We cannot use regular eval since that is disabled, and we cannot use require(...) since those run outside of the IoContext... turns out just adding these scripts as text bindings and passing them to the unsafe eval binding makes it work.

[anonrig]

Many of the wpt test files rely on the need to perform io at the top level scope, as you know we want to be able to run those wpt files automatically as is, without porting those to workerd syntax. This PR is the solution @npaun and I came up with to support the case. We cannot use regular eval since that is disabled, and we cannot use require(...) since those run outside of the IoContext... turns out just adding these scripts as text bindings and passing them to the unsafe eval binding makes it work.

Yes, but this PR doesn't add any of those new tests. It would be helpful if we at least enabled 1 test that uses this new code.

[npaun]

It's not a particularly awesome demo but there is a simple way to check this change is working.

Whereas before urlencoded-parser.any.js's Response tests would fail with:

Error: Disallowed operation called within global scope. Asynchronous I/O (ex: fetch() or connect()), setting a timeout, and generating random values are not allowed within global scope. To fix this error, perform this operation within a handler. https://developers.cloudflare.com/workers/runtime-apis/handlers/

Now they fail with:
TypeError: Non-utf-8 application/x-www-form-urlencoded body.

Which shows we can now run that part of the test code.

[anonrig]

Next time please add a test before merging that asserts the intended behavior.