RFC 141: Change testharness to include late tests (#141)

* Change testharness to include late tests

* Fix formatting issues

Author: jcscottiii

rfcs/testharness_include_late_tests.md

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+# RFC 141: Change testharness to include late tests
+
+## Summary
+
+Currently, "late tests" are
+[ignored](https://github.com/web-platform-tests/wpt/blob/c751476c04678faa79e598683f2a6e94d17cf9e0/resources/test/tests/unit/late-test.html#L10-L17).
+This RFC proposes that testharness should now include them.
+
+## Details
+
+Conside the following test:
+
+```html
+<script>
+window.onload = () => {
+  test((t) => { ... }, 'test 1');
+  test((t) => { ... }, 'test 2');
+  test((t) => { ... }, 'test 3');
+};
+</script>
+```
+
+Currently, only test 1 will be run. The issue is that the testharness
+immediately adds a window load handler that marks `all_loaded = true`,
+and that ends the tests as soon as the first result from the first test
+is processed. (The test runner waits for the first test because
+`Tests.prototype.all_done()` also waits until `this.tests.length > 0`.)
+There were various mitigating corner cases, such as if you started
+the list of tests with a `promise_test()`, that would increment a
+counter that kept the rest of the tests alive. Etc.
+
+The proposal is to change testharness `window.onload` handler to run
+with a `setTimeout(0)` so that all_loaded is only set to true after all of
+the tests are loaded by any `window.onload` handler.
+
+## Risks
+
+This change will expose a few tests that should have been failing but were
+masked by the lack of test coverage - bugs have been filed for those.
+
+Also, several tests that were working around this via various
+means will be cleaned up in the implementation of this RFC.