Tests: Enable test/main/promise.js in mozjs CI job

It just had one inline call to setTimeout in one of the test cases,
which is easily changed to use our local `defer` function instead.
After that, it passed.

While at it, simplify and synchronise the two mock-promise functions.
Not yet worth abstracting I think, but thinking about it.

Ref #1511.

.github/workflows/CI.yaml

@@ -65,6 +65,23 @@ jobs:
       - name: Tests
         run: ${{ matrix.script }}
 
+  # To reproduce SpiderMonkey builds locally:
+  #
+  # 1. Run the build the same way you normally do.
+  #    ```
+  #    nobody@nodejs-isolated:/qunit$ npm run build
+  #    ```
+  # 2. Start a temporary Docker container using the official Ubuntu image from DockerHub,
+  #    and mount the current working directory in it:
+  #    ```
+  #    you@host:/qunit$ MNT=$(basename "$PWD"); docker run --rm --interactive --tty --mount type=bind,source="$PWD",target="/$MNT",readonly --entrypoint /bin/sh ubuntu:focal -c "cd /$MNT;bash"
+  #    ```
+  # 3. Run the following from the Docker container's bash prompt:
+  #    ```
+  #    root@ubuntu-tmp/qunit$ apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y libmozjs-68-dev
+  #    root@ubuntu-tmp/qunit$ js68 test/mozjs.js
+  #    ```
+  #
   sm-test:
     name: SpiderMonkey
     runs-on: ubuntu-20.04

test/main/assert.js

@@ -1,16 +1,11 @@
 function buildMockPromise( settledValue, shouldFulfill ) {
 
-	// Support IE 9: Promise not supported, test should not load polyfil globally to ensure
-	// QUnit works without it, so we create our own thenable with setTimeout.
-	// Support SpiderMonkey: setTimeout is not supported, re-use the IE 9 mock, but
-	// using SM's native Promise support for the deferred callback handling.
-	var defer = typeof setTimeout !== "undefined" ?
-		setTimeout :
-		function( fn ) {
-			Promise.resolve().then( fn );
-		};
+	// Support IE 9: Promise not supported, test MUST NOT load polyfil globally.
+	// Support SpiderMonkey: setTimeout is not supported, but native Promise is.
+	var defer = typeof setTimeout !== "undefined" ? setTimeout : function( fn ) {
+		Promise.resolve().then( fn );
+	};
 
-	// Return a mock self-fulfilling Promise ("thenable")
 	var thenable = {
 		then: function( fulfilledCallback, rejectedCallback ) {
 			defer( function() {
@@ -19,12 +14,10 @@ function buildMockPromise( settledValue, shouldFulfill ) {
 					rejectedCallback.call( thenable, settledValue );
 			} );
 
-			// returning another thennable for easy confirmation
-			// of return value
+			// returning another thenable for easy confirmation of return value
 			return buildMockPromise( "final promise", true );
 		}
 	};
-
 	return thenable;
 }
 

test/main/promise.js

@@ -1,15 +1,18 @@
+// Support IE 9: Promise not supported, test MUST NOT load polyfil globally.
+// Support SpiderMonkey: setTimeout is not supported, but native Promise is.
+var defer = typeof setTimeout !== "undefined" ? setTimeout : function( fn ) {
+	Promise.resolve().then( fn );
+};
+
 // NOTE: Adds 1 assertion
 function createMockPromise( assert, reject, value ) {
 	if ( arguments.length < 3 ) {
 		value = {};
 	}
-
-	// Return a mock self-fulfilling Promise ("thenable")
 	var thenable = {
 		then: function( fulfilledCallback, rejectedCallback ) {
-			assert.strictEqual( this, thenable, "`then` was invoked with the Promise as the " +
-				"context" );
-			setTimeout( function() {
+			assert.strictEqual( this, thenable, "`then` invoked with our Promise as thisValue" );
+			defer( function() {
 				return reject ?
 					rejectedCallback.call( thenable, value ) :
 					fulfilledCallback.call( thenable, value );
@@ -150,10 +153,10 @@ QUnit.module( "Support for Promise", function() {
 		assert.expect( 2 );
 
 		var done = assert.async();
-		setTimeout( function() {
+		defer( function() {
 			assert.true( true );
 			done();
-		}, 100 );
+		} );
 
 		// Adds 1 assertion
 		return createMockPromise( assert );

test/mozjs.js

@@ -43,7 +43,7 @@ loadRelativeToScript( "../test/main/assert.js" );
 loadRelativeToScript( "../test/main/assert/step.js" );
 // Requires setTimeout, loadRelativeToScript( "../test/main/assert/timeout.js" );
 // Requires setTimeout, loadRelativeToScript( "../test/main/async.js" );
-// Requires setTimeout, loadRelativeToScript( "../test/main/promise.js" );
+loadRelativeToScript( "../test/main/promise.js" );
 loadRelativeToScript( "../test/main/dump.js" );
 // Requires setTimeout, loadRelativeToScript( "../test/main/modules.js" );
 loadRelativeToScript( "../test/main/deepEqual.js" );