test(Misc): Add test coverage to the SubrequestsTree

tests/src/Unit/SubrequestsTreeTest.php

+<?php
+
+namespace Drupal\Tests\subrequests\Unit;
+
+use Drupal\subrequests\Subrequest;
+use Drupal\subrequests\SubrequestsTree;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\subrequests\SubrequestsTree
+ * @group subrequests
+ */
+class SubrequestsTreeTest extends UnitTestCase {
+
+  /**
+   * @dataProvider dataProviderStack
+   * @covers ::stack
+   * @covers ::getLowestLevel
+   * @covers ::getNumLevels
+   */
+  public function testStack($input, $expected_count) {
+    $sut = new SubrequestsTree();
+    $sut->stack($input);
+    $this->assertSame(1, $sut->getNumLevels());
+    $this->assertSame($expected_count, count($sut->getLowestLevel()));
+  }
+
+  public function dataProviderStack() {
+    $defaults = [
+      'requestId' => 1,
+      'body' => '',
+      'headers' => [],
+      'waitFor' => 1,
+      '_resolved' => FALSE,
+      'uri' => '',
+      'action' => '',
+    ];
+    return [
+      [[new Subrequest($defaults), 12, new Subrequest($defaults)], 2],
+      [[12], 0],
+    ];
+  }
+
+}