Call $DONE function exactly once per test

Also make a note in CONTRIBUTING.md that `$DONE` shouldn't be called multiple times.

CONTRIBUTING.md

@@ -298,7 +298,7 @@ Consumers that violate the spec by throwing exceptions for parsing errors at run
 
 ## Writing Asynchronous Tests
 
-An asynchronous test is any test that include the `async` frontmatter flag. When executing such tests, the runner expects that the global `$DONE()` function will be called to signal test completion.
+An asynchronous test is any test that include the `async` frontmatter flag. When executing such tests, the runner expects that the global `$DONE()` function will be called **exactly once** to signal test completion.
 
  * If the argument to `$DONE` is omitted, is `undefined`, or is any other falsy value, the test is considered to have passed.
 

test/built-ins/Promise/allSettled/reject-deferred.js

@@ -43,5 +43,4 @@ Promise.allSettled([thenable])
     assert.sameValue(settleds.length, 1);
     assert.sameValue(settleds[0].status, 'rejected');
     assert.sameValue(settleds[0].reason, simulation);
-    $DONE();
   }).then($DONE, $DONE);

test/built-ins/Promise/allSettled/reject-ignored-deferred.js

@@ -49,5 +49,4 @@ Promise.allSettled([fulfiller, rejector])
     assert.sameValue(settleds[0].value, 42);
     assert.sameValue(settleds[1].status, 'fulfilled');
     assert.sameValue(settleds[1].value, simulation);
-    $DONE();
   }).then($DONE, $DONE);

test/built-ins/Promise/allSettled/reject-ignored-immed.js

@@ -40,5 +40,4 @@ Promise.allSettled([fulfiller, lateRejector])
     assert.sameValue(settleds[0].value, 42);
     assert.sameValue(settleds[1].status, 'fulfilled');
     assert.sameValue(settleds[1].value, simulation);
-    $DONE();
   }).then($DONE, $DONE);

test/built-ins/Promise/allSettled/reject-immed.js

@@ -27,5 +27,4 @@ var thenable = {
 Promise.allSettled([thenable])
   .then((settleds) => {
     checkSettledPromises(settleds, [{ status: 'rejected', reason: simulation }]);
-    $DONE();
-  }).then($DONE, $DONE);
+  }).then($DONE, $DONE);