Fix fatal in test run under PHP 8

```
Fatal error: Declaration of PostFilters::setUp() must be compatible with Yoast\PHPUnitPolyfills\TestCases\TestCase::setUp(): void in /builds/wp-plugins/view-all-posts-pages/tests/test-post-filters.php on line 40
```

tests/bootstrap.php

@@ -27,5 +27,8 @@ function _manually_load_plugin() {
 }
 tests_add_filter( 'muplugins_loaded', '_manually_load_plugin' );
 
+// Load base class for compatibility.
+require __DIR__ . '/class-test-case.php';
+
 // Start up the WP testing environment.
 require $_tests_dir . '/includes/bootstrap.php';

tests/class-test-case.php

+<?php
+/**
+ * Compatibility shim for PHP 8 tests.
+ *
+ * Yoast polyfills add return type declaration to `setUp` that isn't supported
+ * before PHP 7.1, hence this workaround.
+ */
+
+namespace View_All_Posts_Pages\Tests;
+
+if ( version_compare( phpversion(), '8.0.0', '<' ) ) {
+	/**
+	 * Class TestCase.
+	 */
+	abstract class TestCase extends WP_UnitTestCase {
+		/**
+		 * Set up the test.
+		 *
+		 * @return void
+		 */
+		protected function setUp() {
+			parent::setUp();
+			$this->_do_set_up();
+		}
+
+		/**
+		 * Set up the test.
+		 *
+		 * @return void
+		 */
+		abstract function _do_set_up();
+	}
+} else {
+	abstract class TestCase extends WP_UnitTestCase {
+		/**
+		 * Set up the test.
+		 *
+		 * @return void
+		 */
+		protected function setUp(): void {
+			parent::setUp();
+			$this->_do_set_up();
+		}
+
+		/**
+		 * Set up the test.
+		 *
+		 * Not setting a return type as implementing methods cannot always do
+		 * so.
+		 *
+		 * @return void
+		 */
+		abstract protected function _do_set_up();
+	}
+}

tests/test-post-filters.php

@@ -5,10 +5,12 @@
  * @package View_All_Posts_Pages
  */
 
+use View_All_Posts_Pages\Tests\TestCase;
+
 /**
  * Content-filter test case.
  */
-class PostFilters extends WP_UnitTestCase {
+class PostFilters extends TestCase {
 	/**
 	 * Text for each page of multipage post.
 	 *
@@ -40,7 +42,7 @@ class PostFilters extends WP_UnitTestCase {
 	 * Not using `setUp` because Yoast polyfills add a return type for PHP 8
 	 * that isn't supported before PHP 7.1.
 	 */
-	protected function _create_post() {
+	protected function _do_set_up() {
 		static::$post_id = $this->factory->post->create(
 			array(
 				'post_title'   => 'Pagination Test',
@@ -55,8 +57,6 @@ class PostFilters extends WP_UnitTestCase {
 	 * Test retrieving page 1 content.
 	 */
 	public function test_view_page_1() {
-		$this->_create_post();
-
 		query_posts(
 			array(
 				'p' => static::$post_id,
@@ -76,8 +76,6 @@ class PostFilters extends WP_UnitTestCase {
 	 * Test retrieving page 2 content.
 	 */
 	public function test_view_page_2() {
-		$this->_create_post();
-
 		query_posts(
 			array(
 				'p'    => static::$post_id,
@@ -98,8 +96,6 @@ class PostFilters extends WP_UnitTestCase {
 	 * Test retrieving page 3 content.
 	 */
 	public function test_view_page_3() {
-		$this->_create_post();
-
 		query_posts(
 			array(
 				'p'    => static::$post_id,
@@ -120,8 +116,6 @@ class PostFilters extends WP_UnitTestCase {
 	 * Test retrieving "view all" contents.
 	 */
 	public function test_view_all() {
-		$this->_create_post();
-
 		query_posts(
 			array(
 				'p'        => static::$post_id,
@@ -137,7 +131,7 @@ class PostFilters extends WP_UnitTestCase {
 			$content = get_the_content();
 
 			foreach ( static::$pages_content as $page => $text ) {
-				$this->assertContains( $text, $content, "Failed to assert that content contained page {$page}." );
+				$this->assertStringContainsString( $text, $content, "Failed to assert that content contained page {$page}." );
 			}
 		}
 	}