Fix tests deprecation

Author: aristath

tests/phpunit/test-class-content-activity.php

@@ -52,7 +52,7 @@ public function test_get_points_for_nonexistent_post(): void {
 	 */
 	public function test_points_decay_over_time( $days_ago, $expected_ratio ): void {
 		// Create a test post.
-		$post_id = $this->factory->post->create(
+		$post_id = $this->factory()->post->create(
 			[
 				'post_content' => 'Test content',
 				'post_status'  => 'publish',
@@ -96,7 +96,7 @@ public function age_decay_provider(): array {
 	 * @return void
 	 */
 	public function test_points_caching(): void {
-		$post_id = $this->factory->post->create(
+		$post_id = $this->factory()->post->create(
 			[
 				'post_content' => 'Test content',
 				'post_status'  => 'publish',
@@ -125,7 +125,7 @@ public function test_points_caching(): void {
 	 * @return void
 	 */
 	public function test_different_activity_types(): void {
-		$post_id = $this->factory->post->create(
+		$post_id = $this->factory()->post->create(
 			[
 				'post_content' => 'Test content',
 				'post_status'  => 'publish',