Unblock test promise chain when done() is called early.

This makes it possible to end a promise_test by calling Test.done().
Previously, that would work for a single test, but would get any
follow-up test stuck waiting for the global promise chain to resolve.

Author: pwnall

testharness.js

@@ -527,11 +527,14 @@ policies and contribution forms [3].
             tests.promise_tests = Promise.resolve();
         }
         tests.promise_tests = tests.promise_tests.then(function() {
+            var earlyDonePromise = new Promise(function(resolve) {
+                test.add_cleanup(resolve);
+            });
             var promise = test.step(func, test, test);
             test.step(function() {
                 assert_not_equals(promise, undefined);
             });
-            return Promise.resolve(promise)
+            return Promise.race([promise, earlyDonePromise])
                 .then(
                     function() {
                         test.done();