Improve async tests

testing.async(...) is pretty lame. It works for simple cases, where the
microtask is very quickly resolved, but otherwise can't block the test from
exiting.

This adds an overload to testing.async and leverages the new Runner
https://github.com/lightpanda-io/browser/pull/1958 to "tick" until completion
(or timeout).

The overloaded version of testing.async() (called without a callback) will
increment a counter which is only decremented with the promise is resolved. The
test runner will now `tick` until the counter == 0.

src/testing.zig

@@ -410,21 +410,46 @@ fn runWebApiTest(test_file: [:0]const u8) !void {
     page.js.localScope(&ls);
     defer ls.deinit();
 
-    var try_catch: js.TryCatch = undefined;
-    try_catch.init(&ls.local);
-    defer try_catch.deinit();
+    {
+        var try_catch: js.TryCatch = undefined;
+        try_catch.init(&ls.local);
+        defer try_catch.deinit();
+
+        try page.navigate(url, .{});
+    }
 
-    try page.navigate(url, .{});
     var runner = try test_session.runner(.{});
     try runner.wait(.{ .ms = 2000 });
 
-    test_browser.runMicrotasks();
+    var wait_ms: u32 = 2000;
+    var timer = try std.time.Timer.start();
+    while (true) {
+        var try_catch: js.TryCatch = undefined;
+        try_catch.init(&ls.local);
+        defer try_catch.deinit();
 
-    ls.local.eval("testing.assertOk()", "testing.assertOk()") catch |err| {
-        const caught = try_catch.caughtOrError(arena_allocator, err);
-        std.debug.print("{s}: test failure\nError: {f}\n", .{ test_file, caught });
-        return err;
-    };
+        const js_val = ls.local.exec("testing.assertOk()", "testing.assertOk()") catch |err| {
+            const caught = try_catch.caughtOrError(arena_allocator, err);
+            std.debug.print("{s}: test failure\nError: {f}\n", .{ test_file, caught });
+            return err;
+        };
+        if (js_val.isTrue()) {
+            return;
+        }
+        switch (try runner.tick(.{ .ms = 20 })) {
+            .done => return error.TestNeverSignaledCompletion,
+            .ok => |next_ms| {
+                const ms_elapsed = timer.lap() / 1_000_000;
+                if (ms_elapsed >= wait_ms) {
+                    return error.TestTimedOut;
+                }
+                wait_ms -= @intCast(ms_elapsed);
+                if (next_ms > 0) {
+                    std.Thread.sleep(std.time.ns_per_ms * next_ms);
+                }
+            },
+        }
+    }
 }
 
 // Used by a few CDP tests - wouldn't be sad to see this go.