test: Silence Karma warning over node:stream from RTS

[rschristian]

We've had this noisy warning since we landed pipeable streams in RTS:

Error: R] Could not resolve "node:stream"

    node_modules/preact-render-to-string/dist/stream-node.module.js:1:28:
      1 │ import { PassThrough } from 'node:stream';
        ╵                             ~~~~~~~~~~~~~

  The package "node:stream" wasn't found on the file system but is built into node. Are you trying to bundle for node? You can use "platform: 'node'" to do that, which will remove this error.

Our tests don't actually use the package.json for module resolution, so preact/compat/server is assumed to be preact/compat/server.js, rather resolving to preact/compat/server.browser.js which removes the Node-only pipeable stream stuff. Explicitly referencing it here seems fine for the moment, it's the only place we test it and we're very explicitly in a browser test.

[coveralls]

coverage: 99.618%. first build
when pulling 17fd092 on test/node-stream-warning
into 2e85690 on main.