Add an RFC for add_top_level_completion_handler

rfcs/add_top_level_completion_handler.md

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+# RFC XXX: add a `add_top_level_completion_handler` to testharness.js
+
+## Summary
+
+Make it possible to attach a completion handler that only runs if no
+`RemoteContext` is listening for results from a `Tests` object.
+
+## Details
+
+Currently, if you have a completion handler which outputs to `console.log` (or
+something else similarly global), then you'll get output from both the top-level
+test (i.e., the document loaded by the test URL) but also from any other window
+opened or any nested frames.
+
+This is fundamentally the cause of
+https://bugs.webkit.org/show_bug.cgi?id=259409, as we're considering the test as
+having to run to completion when a completion handler first runs (and it seems
+to be somewhat nondeterministic which order they run in, but that just adds
+flakiness to the already potentially bad race).
+
+This RFC proposes adding an `add_top_level_completion_handler` API to
+testharness.js which will only run if no `RemoteContext` is listening for a
+`Tests` results.
+
+Specifically, this requires modifying `RemoteContext` to send a message that
+causes the remote context to save the "is remote context" state, to then be able
+to avoid running the top-level completion handler.
+
+Currently, this technically isn't needed: a completion handler can simply guard
+itself with `if (window !== window.top || window.opener !== null) return;`,
+however if at some point we make it possible to run `fetch_tests_from_window` on
+a window opened with `noopener` this then becomes a necessity.
+
+## Risks
+
+This is a simple API addition to `testharness.js` that allows
+`testharnessreport.js` to ensure they only get completion handlers they care
+about; this should pose only the risks associated with any modification of
+`testharness.js`.