Fix loading of WPT META scripts

[tschneidereit]

WPT tests can load helper scripts using special comments at the top. These scripts need to be loaded as though using a <script> tag, which doesn't create a new lexical scope, and hence causes let and const bindings to be visible in the global scope.

Previously, to simulate this behavior using eval, the WPT harness replaced let and const with var. While this worked surprisingly well, it doesn't always work.

This commit instead introduces an evalScript function available to the harness, which evaluates in a way more closely matching that of script tags.

[tschneidereit]

@andreiltd this should hopefully make the tests we talked about in #200 work

[andreiltd]

Hey @tschneidereit. That's a great improvement, thanks! I gave it go on the form-data branch and indeed the Not Defined error is gone. 🎉 However I'm getting this error when I try to run the tests:

1: WPT server stderr: 127.0.0.1 - - [09/Feb/2025 10:31:49] code 400, message Bad request syntax ('1D9')
1: 127.0.0.1 - - [09/Feb/2025 10:31:49] "1D9" 400 -

I will need to take a closer look if that's linked to FormData changes or something else although I can see the other tests are failing in a similar way. For example:

1: Running test fetch/api/redirect/redirect-upload.h2.any.js
1: Sending request to http://127.0.0.1:7676/fetch/api/redirect/redirect-upload.h2.any.js
1: WPT server stderr: 127.0.0.1 - - [09/Feb/2025 10:31:30] code 400, message Bad request syntax ('4')
1: 127.0.0.1 - - [09/Feb/2025 10:31:30] "4" 400 -

[andreiltd]

I think I figured this out. The 400 error is thrown when I'm not setting the Content-Length header for the Request with ReadableStream body. After fixing the tests are passing 🎉 .

[tschneidereit]

I talked to @guybedford, and while he doesn't have the bandwidth to do a proper review, he gave his okay to merge this as-is, given that @andreiltd gave positive feedback. This also only touches test code, nothing we ship.